It's not that sillly imho...
In fact, it is very logical that they must be stretched.
Scenes are shared and shown to your contacts. But this means that they need to work on all screen resolutions going from 640x480 to 2000x2000 (or whatever hi-def screen size someone has). Also, everybody has different conversation window sizes; some have the convo window very narrow, some have them maximized over their entire desktop area.
To accomodate all this you have 2 options: either allow for a limited maximum size of the background and stretch it to fit all resolutions, or always send a 2000x800 big photograph (which would be many many megabytes big!!!!) to all of your contacts.
But ofcourse sending a massive big photograph is no option at all. It is impossible to send tens of megabytes to all your contacts each time. It would simply be way too slow and thus not work for an IM program.
Hence the (logical choice of) stretching.
Scenes can be made nice if you put some thought in it and hence I find them not restricting at all.
Don't look at it as pictures which need to be pixel-perfect! Simply make images which wont look bad when they are streched.
As with many 'restrictions': use your fantasy and imagination instead, aka be creative.
So, restrictions like this don't need to be that limitating at all if you put some thought in it. On the contrary, although it might seem silly and annoying at first, they actually force you to be more creative and original. Make (or copy) some nice color gradients or whatever art you fancy instead of trying to put some pixel-perfect object in the scene.