I downloaded this picture: http://www.forestwander.com/wp-content/original/2009_07/Elakala_Waterfalls_pub11.JPG Ristretto 0.3.6 does not seem to be able to render it correctly with all the details contained in the file, see attachment for comparison with how gThumb renders the image.
Created attachment 5187 Comparison of how gThumb and Ristretto 0.3.6 displays the same image. Attachment failed, trying again. -_-
Created attachment 6055 Comparison I've just compared ristretto, eye of gnome and viewnior. The results are pretty similar, but you must disable the option "Limit rendering quality" on preferences, which I guess was enabled in your case.
Closing this as it seems was just a false positive. If not, please reopen.