When I highlight several images in a folder in Thunar and press enter, Ristretto opens a viewer which can scroll through all of the images in a folder. On the other hand, when you highlight several images and then right-click and select "Open in Ristretto", it opens a viewer which can scroll only through the selected images (which was the original intent in my first case). Can you change Thunar to pass in the selected filenames to Ristretto so these both behave like the second case? This is not a bug in Ristretto as far as I can tell because I have performed this same test in other file managers without the same problem.
Yes, only the last selected item is opened, thus Ristretto opens only one image. The workaround is to select the files you want, right click on one of them and select Open With "Ristretto Image Viewer" from context menu. *** This bug has been marked as a duplicate of bug 2487 ***