Therefore it must fit these criteria:
1) A joke that is not solely a visual one, and that can be described in words.
2) A joke that does not rely on knowledge of cat macros to be amusing (ceiling cat, cheezburger, flavor, etc.)
3) A joke that is not one of the clichés of cat macro-dom, such as ceiling cat, cheezburger, etc., so that my protagonist can find it funny.
I have spent the last hour or so looking through cat macro sites (oh god), but this is surprisingly difficult to fulfil. I've been using my Dad, who is unfamiliar with them, to judge if they are funny to a n00b, but apparently they are not.
I googled 'lolcat' and 'cat macro', and looked on the image search for both of those, too. I'm in the UK and so is my character - my supervisor is from India. I don't think that matters too much though.
(PS. Mods, I'm not sure if this is the right sort of question for Little_Details, but I can't think of a better place to ask. If it's not ok here, please let me know and I can delete it!)