Hi baberjaved, perhaps you mean this: you link a thumbnail to the big image. The big image appears on a blank page - without any title, alt-attribute or anything. Google will not index those blank-pages ( if my tests are correct and actual ). In the image-search the link shows to your page with the thumbnail. This is the page that could rank. Its the same if you use an ajax-gallery.
There is no problem if you have lots of these "blank sites with big images".
But, as tedster said: the algorythms for image-search and organic-search are completly different. I have about 400 images on the first page of images-search - but only 3 or 4 of these can also be found on page one of the organic search (but - of course - many of these are on the first page because of universal-search box with images ;-) Best wishes and good luck, Martin
Images appear to self-sort over time, probably based on click feedback, which indicates relevance and quality for a given query. This sorting dynamic may be affected by more image results showing up on the general serps though. Links to image pages appear to be important for credibility, and thematic structure of pages helps, perhaps as a measure of relvance/authority.
It seems to help to use a pics label for safe searches used in schools and business to be slightly more competitive.
Incidentally, I noticed traffic from images.google.com dropped 98% on May 6 for a photo site, without an overall drop in traffic-- referrals may have shifted to google.com/imgres though not as abruptly as the drop.