Science tattoos. Funny.
If I made such a tattoo, I'd choose 'universal property' picture
for I always forget how it looks when I need it.
Or, possibly, I would define left and right subgroup cosets
gN and Ng
on my left and right hand respectively, for I incessantly confuse one with another. (And generally, I often have problems with remembering where is 'left' and where is 'right' when it comes to math definitions, whilst in casual life I never have such problem.)