@NIGHTKILLER05 I'm not at my device right now to test it but I think quoting the actual "${0%.*}" should do it. You'll need to use "double-quotes", since 'single-quotes' prevent ${variable_expansion}. Since it's inside the (parentheses), I think it's okay to have the quotes inside the other quotes and you shouldn't have to \"escape them\". Try:
ln -sf "/home/pi/.emulationstation/downloaded_media/ports/screenshots/$(basename "${0%.*}").png" /opt/retropie/configs/ports/doom/launching.png