Hmm, on the Mac the edit button is disabled when there is no active image file used. I'll have to take a look at it on Windows, there's no reason for it to behave differently.What is the intended behavior when the "Edit" button is clicked while the "Use default" checkbox is selected? I tried to make it work manually, but was not successful (texture was not loaded after saving) and now I'm just wondering wether the "Edit" button should be grayed out in this state.
A feature I hope we can do for the next release is to have OR create an appropriately-sized blank image for the editor when there is no current image selected.