I've read this post Make google index the actual image not the thumbnail but my case is a little different.
I have over a thousand images, in many different directories, with many different names, and in most cases the ones with full resolution are the ones indexed by google (bing is even better), but in some cases the thumbnails are the ones indexed, unfortunately.
I need to have 2 separate images because we add a watermark to all our full-sized images since in the past i caught competitors stealing and using our images of our custom products, therefore, scaling down an image with a watermark would not look good.
The images, when clicked on, lead to a page that displays the full image, with product description and not only the image itself.
90% of these pages are indexed by google and bing, and about 60% of the images (but still going up every day).
Thumbnails are named asd-asd-asd_small.jpg (example) while full resolution pictures are named asd-asd-asd.jpg.
I use A1 Sitemap Generator for all my sitemaps, including images, but since i'm talking about over a thousand images, is there a practical way, easy to implement that would give priority to the full size images on indexing?