I just figured out how I can copy and paste smaller parts of the bmp drawing layer:
selection rectangle to choose what you want
goto menu and press Copy
place the selection rectangle somewhere else and goto menu to press paste
The only issue I face is, that I do not know how to keep the pasted drawing the same in size as the original. How can I keep the same ration between Y and Z of the selection rectangle??
Several approaches are possible in order to keep the image (the one you want) constrained to its original size
. Copy the part of the image that you want and paste it from its original place/frame into another frame: just select the part that you want with the Selection Box (V), click Copy (CTRL C), go to the other frame, click Escape (ESC) and Paste (CTRL V). The Escape Key will delete the Selection Box and prevent Pencil2D from not pasting the image.
. Copy the part of the image that you want and paste it within the same frame: just select the part that you want (V), click Copy (CTRL C), and with the Select tool selected (V) (the Selection Box will still be visible on the canvas) make another Box Selection bigger than your previous one, and Paste it (CTRL V). You will get the image scaled to its original. Click Escape (ESC) if you want to deselect the Box Selection.
The point here is to make the Selection bigger than the first one, this way Pencil2D will keep the size of the image constrained to its original