Hmm that's strange, I'm able to use "load putrid_s4nd.p8.png" from the pico 8 command line, at least on windows.
Does it work if you re-save the p8.png as a regular .p8 file (by loading it in pico 8 and calling 'save' again on windows or linux, wherever it currently works for you) and then pass that new .p8 file to your raspi?