Image Size ...Wish

If want image as background, you can use the decoration image, located on decoration image section of the menu.
I suggest do at the end of your template, when you finish, load image as a decoration,  move to the background with z order.
Anyway I think will be better a feature for load images as background.

....It would be cool to have the possibility to cut also the holes into wallpaper/background...

because if you want to set a complete background picture at the moment it isnt't possible with holes

Duet how the software is designed and what you request, the amount of calcs for do that per page are really higher and require a lot of code.
I can confirm I not go in this direction and also on skinable software direction.
There is a lot of features more useful in terms of controllerism see the light across 2019, and all the focus will be on that.
Hope you understand.
You can do this with adobe illustrator, or the free  and super nice software Gimp:    https://www.gimp.org/

no problem.... was only an idea  ? 

...thanks for the tip with gimp think that should work as *.png with transparence ? 

Yes, work with transparent PNG.
make screen capture of your template, then load on Gimp or any pro graphic editor as the first layer, play with transparency of layers, then load your design, cut the holes, save as transparent png.
Save the original work in illustrator or gimp native format, so you have the file for future changes.
That should work.