>>22918119I know why and am more American than the other one
The preposition "on" is used when something is "on top of" something else and the image background is serving as an analog for a paper or chalkboard or something
So the pic of the guy is on the bottom
Whereas "at the bottom" is actually also correct and you can use it in this situation since it denotes a location, but you would be more likely to use it when referring to a contiguous picture of multiple people in one background/location
Whereas here it creates a bit of possible ambiguity in that you would normally know what he is saying but there's a possibility he could be referring to the irl location the bottom man is in since the top man is at the beach, ie he could possibly be saying he is also in a white void near this guy
But both are correct and I might say either
In fact, I might say me IN the bottom, which would give a sense that I am really in the picture and really am the guy. In works both because it's inside a picture and because it's inside a metaphorical depiction of an abstract concept.
So I could be saying I'm the guy, I have dried balls, OR even that I have my dick in the bottom guy's ass with the double entente then that I am referring to him as a "bottom" (noun) a man who takes it in the ass OR a British or American parental euphemism for an ass.
Or any combination of all three possible meanings.
I hope you find this lesson on the contextual expressiveness of English prepositions to be sufficiently edifying.
