pasting over bitmaps will result in squashing

[galap-1]

If you paste a bitmap (for example a scenery object bitmap) into an existing slot, the bitmap will be squashed to the size of the bitmap that previously used that frame. If you make a new frame, and assign that frame the pasted bitmap it will fix the issue.

[aaronfreed]

It's also possible to fix by changing the scale factor and/or X/Y origin points and then changing them back (I'm not sure which of those fixes it, or if it's necessary to do them both).