Regarding the randomness of Google Image Search indexing

Has anyone here had much luck in optimizing for Google Image Search? Or are chances of having an image indexed ultimately still more random than targeted?

I have more than 6 thousand images in my website galleries (not counting thumbnails or other layout images) and I've followed every step I've heard of to optimize them for search. Each image has it's own page with the name of the image in the page filename, a page title to describe the image, a matching text title right above the image, an alt tag and title tag with the image title, and all images are large resolution. The thumbnails that lead to each full image page also have a descriptive text title and matching alt/title tags. My domain is about 5 years old, PR5, Alexa 40k rank, and has well over 4,000 URLs indexed, and the content has always been images. Many of the gallery pages are PR2-3 and rank on the first web results page for terms matching images on the page.

Despite all this, Google Image Search only returns a little over 500 images for my website, and most of them are from layout graphics or sections I'm not targeting. ~_~;

This is rather frustrating as images make up the bulk of my content, and that's what I'd like to get searches for. There's not a lot of text on my site, yet I still get more Google web searches than image searches. Is there possibly something I'm doing wrong? The reason I decided to ask is because I recently noticed a public domain photo site I visit has more than 4,500 images indexed, roughly 80-90% of their stock from what I can tell. They use mostly the same optimization steps I do, and have very little text.

If you have any experience or advice in this area, I'd love some insight. Thanks!