I suggest that a mechanism could be made so that when a skin creator wants to check how an image works on his Messenger, he doesn't have to restart Messenger to see the changes. I mean, for example if he changes an icon, he should be able to simply close the window and when he reopens it, the icon is changed. This would be very helpful for all skin creators. I know it may be difficult to make but it is just a suggestion.

that's a good suggestion indeed. Right now it's not been done fo stability reasons but I still plan to do somehing about it in the future.