Now that I am back to playing with Wayland/Sway, I found the best solution is what @etheling was doing...using imv as the image viewer. The difference is that I am using runcommand-onstart.sh to display the loading screens so I don't have to mess with editingruncommand.sh. The key is to NOT name you launch images *-launching.png (I use *-launch.png) as this will invoke feh via runcomand.sh.
#!/bin/bash ROMPATH="$3" ROM_BN_EXT="${ROMPATH##*/}" ROM_BN="${ROM_BN_EXT%.*}" LAUNCHIMAGE="/home/pi/RetroPie/roms/$1/images/$ROM_BN-launch.png" imv-wayland -f -s full -x "$LAUNCHIMAGE" & &>/dev/null IMG_PID=$! sleep 4 imv-msg "$IMG_PID" "q"