Seriously? This is the kind of question that we should not need to ask. Inquiring whether it is licit to use a baptismal font to sit on is like asking whether it is right to stretch out and sleep in a pew or play Frisbee in the sanctuary. It seems to be so self-evident that such a usage for a holy item is not permitted that one must suffer from an extreme deprivation of common sense to think otherwise. Will we find any specific legislation in any liturgical manuals that say "do not sit on baptismal fonts"? No. The practice is so absurd that probably the great canonists and liturgical writers of old never thought of proscribing it. Unfortunately, with the advent of modernity, this unfortunately is an issue we need to address. The picture you see above was not something I Googled, but a picture taken by a friend of mine at a parish out west prior to the beginning of Mass. Yes, this is happening, and it needs to be addressed, even if the ancient canonists never thought in a million years that this would be an issue. Yet even if we can't find a "Thou shalt not sit on the font", there are well established principles of usage relating to liturgical items and the font in particular such that we can say with confidence and authority that, NO, you may NOT sit on the baptismal font.
Interestingly enough, most legislation on the usage of baptismal fonts comes from the early Middle Ages, while legislation on their construction comes from the Tridentine period. The use and construction of fonts became so formalized by that time that nothing more was said about them for centuries. The current 1983 Code of Canon Law is silent on the matter, and therefore, we must assume that the prior legislation and customs are still in force.
Let us begin by examining the usage of the font.
In the early Middle Ages, to combat superstitions of recently converted pagans surrounding the proper uses and efficacy of Holy Water, the Church legislated on several different occasions that baptismal fonts were to be used solely for performing baptisms. The constitutions of Bishop Poore of Sarum (Salisbury, c. 1217) and of St. Edmund of Canterbury (1236) combated the abuse in England as did the Councils of Tours (1236), Trier (1238), Fritzlar (1243), and Breslau (1248), on the Continent, all of which stipulated the exclusive use of the font for the purpose of baptism, and nothing else. Presumably, this would include placing one's arse on the font to use it as a seat. Although of a provincial nature, these Councils and synods reflect the universal practice of the Church and have never been countermanded or repealed by any subsequent act of Magisterium. They thus constitute a moral norm for approaching questions of the appropriate usage of the font and should preclude any profane usage.
In order to preserve its exclusive usage for baptism, the traditional Roman Ritual stipulated (Tit. ii, cap. i, 28-30) that the font should be railed off; it should have a gate fastened by a lock; and should be adorned, if possible, with a picture of the baptism of Christ by St. John. The purpose of the locking up of the font was precisely to prevent people who wandered into the Church throughout the day from using it in any profane manner, or in other words, to ensure that it was used only for baptisms and nothing else. If the Church taught that the exclusivity of using the font for baptisms only was so important that its most authoritative ritual book stipulated it should be locked up behind a gate, we can easily discern the mind of Church on the question of people using the font to sit on.
In the Middle Ages, for example, holy water was held in such respect that it was not even taken from the font unless by means of an aspersorium or holy water sprinkler, attached by a small chain. This reflected the profound mystery attached to this sacramental, which in the Sacrament of Baptism becomes the sacred and life giving sign of our regeneration.
Coming to the issue of construction of the font, we have legislation from the Tridentine period. In the rules prescribed by St. Charles Borromeo for the construction of fonts in the Diocese of Milan, we see actual legislation on the construction of fonts. While St. Charles' intention here is to describe the material construction of fonts and not their use, note his care that nothing profane be depicted on the font:
"Heretofore we have treated of the sacristy and several other things, let us now speak of the vessel intended for holy water. It shall be of marble or of solid stone, neither porous nor with cracks. It shall rest upon a handsomely wrought column and shall not be placed outside of the church but within it and, in so far as possible, to the right of those who enter. There shall be one at the door by which the men enter and one at the women's door. They shall not be fastened to the wall but removed from it as far as convenient. A column or a base will support them and it must represent nothing profane. A sprinkler shall be attached by a chain to the basin, the latter to be of brass, ivory, or some other suitable material artistically wrought."
The purpose of keeping the font of highest material quality and decoration was meant to safeguard the Church's teaching on the redemptive efficacy of the sacrament, as opposed to the merely symbolic value attributed to it by many of the Reformers. If St. Charles insists that nothing profane may be represented on the font, what would he say about subjecting the font to profane usage?
In case these appeals to traditional norms and legislation are not sufficient, it could also be stated that all references to baptismal fonts in current Church legislation do so only in the context of discussing baptism, which suggests strongly that it is the performance of the Sacrament of Baptism alone that is the appropriate usage of the font. 
The general principle throughout all of Catholic history when dealing with these questions is that holy things are for holy uses. A secular or profane use is any use that is common place, or otherwise divorced from the sacred purpose of the item. Holy oils aren't used for cooking. Communion wafers, even if not consecrated, are not snacked on as munchies; pews are not for sleeping, and baptismal fonts are not for sitting on. If you come into Church late and there is no seating available, it is better to stand or even sit on the floor than sit on a font. At least the floor is meant for profane uses - sitting or standing. A font, on the other hand, is for sacred uses.
All of Christian tradition cries out that sitting on baptismal fonts is wrong; we have documented here why it is wrong, but if you really needed me to tell you this, then I suggest you go back and do some serious studying and praying.
 For example, see Rite of Baptism for Children, 1973, Sec. III