Import aseprite sprite into Pyxel Edit tilesheet

Hi, I'm really new to Pyxel Edit and I have some sprites from Aesprite I would like to import into Pyxel Edit's tilesheet tool. I was able to import a png as separate file, but when I copy/pasted it onto my tilesheet I could actually figure out how to attach it to a tile. It just floated there, but never became part of an indexed tile.