I have two suggestions:

Also scan for an icons subdirectory in the project's own directory.
This will allow one to more easily use (and more importantly backup)
ones own icons. Instead of requiring the user to dump his icon with
all the others in the program directory.

Drag-drop images can also prompt you if you want to copy the image to
a image subdirectory. The ideal would be to simple drag and drop an
image from a website to wikidpad and automatically save it locally.
OK, maybe this will move it slightly towards being like EverNote. But
with wiki markup.