When viewing a folder of images (let's say 4000x3000 in size) the thumbnails are 178x178. People have gotten used to right-clicking on web images and choosing "Save As" even if they shouldn't, such as in this case where that will download a 178x178 low-res image and not the real image. I have (idiot) clients who do this all the time. Yes, I know I can change the url to dl=1 instead of dl=0, but that also prompts them to "download" it to their Dropbox and the problem persists. People are idiots.
Make your own context-sensitive menu with right-click so that the Save As saves the real image/file and not the thumbnail -- that's what Google Drive does. I can't imagine any case where anyone would want to save one of the thumbnails.
By the way, this isn't isolated to images. If you right click and choose Save As with a PDF file it saves the stupid thumbnail image and not the pdf file.