Instead of unconditionally setting the panel's background when an image file URI gets dropped on a panel, ask the user whether to create a launcher for that URI (like for non-image URIs) or to use the image as the panel's background. A number of users have seen their panel's background changed unexpectedly due to unwanted drop of an image file over the panel, sometimes seeing this as a panel's bug. Also, this behavior is inconsistent with dropping a URI pointing to any other type of file, where it would create a launcher for it. Consequently, and as setting the panel's background doesn't seem like a so common task it ought to be super fast, ask the user what to do when receiving an image URI drop.