Not that good at GIMP, but I'm having an issue. I'm trying to copy from a transparent image to another, however it only copies the part that's not transparent. How can I make it also select and copy the part that is transparent, because I have half-opaque pixels there which it's not copying.
What is probably going on is that the target image is not in RGB mod, rather in indexed mode. (It will be so if its file in disk is indexed, like a GIF or an indexed PNG).
GIMP does not represent semi-transparency levels on indexed images - change the target image to RGB mode in
image->mode->RGB before pasting your contents there.
If afterwards you need to export the image to the same file format, you may want to manually convert the image back to indexed mode so that you can fine-tune the results before exporting.