Couldn't find this anywhere on the net and kinda needed it so am making a note of how to do it here.
The issue is that I wanted to make my autohotkey program change the image that the mouse hovers over (within it's own GUI). No third-party component or dll needed, just a slight modification to the mousemove tooltip in the autohotkey manual.
Thought I'd put this down as it took me ages to search the Internet for this solution. In the end, I found it hidden away in the Autohotkey help file and not under the gui events...
The issue here is if you want to do something when a user resizes your program.
If you search and search, you'll find that GuiClose responds to when you close the app, and GuiSize will run when you resize the app.