This'll be in the next "official release," but I got a feature request for this so
here's an interim patch(edit: feature now included in all versions as of v1.50) which adds some functionality: File reloading!
Normally if there are changes to your images directory while REXPaint is open, to see all those changes in RP's browser you'd have to restart the program. Now there's a button at the bottom of the browser, "R" for reload. Just click that and everything will be reloaded to reflect the current state of the file system (you can also use a new hotkey: Ctrl-Shift-r).
You'll get a warning if you have any unsaved progress or images, since of course doing this will erase that. The reloading process will do its best to restore the original state of the browsing window, such as selected image, visible images, last selected image, and folder collapse states, but of course this isn't always possible since it could encounter missing files or different folder structures.
Note that as per other patches since the last official release, this one needs to update your skins.xt file since a new entry was added there for another feature from an earlier patch.