I notice that quite often the same image is not matched across all sites.  The merge option is great once these are identified but identifying them is the problem.  Would it be possible to introduce a thumbnail sort by similarity or even filename?

Thanks for your suggestion. Took some time, but this feature is included in new version released today 1.6.6. In this sort mode program will display only pairs of files that are very similar sorted by update date. This helps a lot to get correct "iStockphoto upload" suggestions.