You have a few options.
Either find all div's with class thumbnail_select by using all("div[class='thumbnail_select']").count
But this is an awkward way of doing it since it looks for the div and not the images.
A better way would to be to look for all images using all("img").count
as long as no other image is present on the page.
If neither of these works either the problem might be that your page is not loaded when you start looking for the images. Then just simply put a page.should have_content
check before the image count to make sure that the page is loaded.