How to insert image to a mask layer?

[Henry]

Thanks for this tip. But here they use a different approach without a mask layer. This would work for plane covers but I also have templates with a curved cover. Here the used perspective tool would not help.

The special thing in the PS tutorial is they open the mask layer in a different (plane) view. Here the cover image can be easily inserted. Then they switch back and the perspective mask layer is automatically applied.

I feel this would be also possible in Gimp because the cover layer and mask is shown too. But so far I couldn't find this special function described before (don't know how it is called so I'm unsure what to search for).