I know this is an older thread but it really saved me! I downloaded GIMP (latest version is currently 2.10.34) and by exporting a PNG, Turning all checks off and compression level to 9 (max) and then pixelformat to 8bpc RGB. With a small filename the image will appear in the UI Flow workspace and on the M5 stick C Plus.

I couldnt get Photoshop, Aseprite and even Paint to get anything in UI flow. But GIMP worked just fine.
Hope this helps someone else down the road.

I know this is an older thread but it really saved me! I downloaded GIMP (latest version is currently 2.10.34) and by exporting a PNG, Turning all checks off and compression level to 9 (max) and then pixelformat to 8bpc RGB. With a small filename the image will appear in the UI Flow workspace and on the M5 stick C Plus.

I couldnt get Photoshop, Aseprite and even Paint to get anything in UI flow. But GIMP worked just fine.
Hope this helps someone else down the road.