It works for Windows and Linux desktops where dragging and dropping is functionally possible, but not for raspi, where splore is launched by a frontend like Emulationstation (common for raspi handheld consoles, arcade cabinets, etc). When I add the file to the /carts dir, it doesn't appear in the File menu in splore either. And, when I try to manually load the .png from the commandline, PICO-8 says "could not load putrid_s4nd.p8.png"
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?