pic names for google images????

I was wondering if it is worth naming your pics to place better in google. It doesn't seem to work. Say you have a pic of a chick sucking black dick. You name the file chick sucking black dick.jpg

you would think it should tell google what the pic is about and should rank higher but it doesn't. why????

you get results like this...
120820‑cute‑amateur‑w.jpg
sexy_blond_white_slut_sucks_dick.jpg
2cd7d16f66293bc65613856dc4635f2d.jpg
preview.flv.jpg
t1070105x2d60b2.jpg

so why even bother naming your files??? It seems to me it doesn't seem to matter.