The thumbnail provided by tumbler (and used in the icon_view) could be used as appicon as well. This way e.g. multiple instances of ristretto could be distinguished more easily. (Btw, Gimp does this.) When no image is open the normal appicon could be used. I guess the concerns here are whether updating the appicon while switching through images is costly in any way (cpu,ram) and at what resolution to export the thumbnail (iirc tumbler provides different sizes). For the panel a smaller size would suffice, but for the tabwin something between 48 and 128px would be good.
Implemented in commit: http://git.xfce.org/apps/ristretto/commit/?id=90f831384b05a30b62a170415771579af6524109