Only other thing I can come up with is this thread here at pixles, not sure if any of it (partially) applies to your issue though:
I did notice something else in your posted screenshot: The map you circled (next to the custom button) is not there for pointing RT to your images, it is for pointing RT to profiles that can be used.
Everything for browsing files and folders to get to the images is done from here:
I think this one might be too obvious, but just in case: Did you check if all the image formats are checked in the preferences? This section:
