I was having problems uploading a 128x128 unanimated png (went straight to "This emote is no longer available") that only went away after re-encoding the image. The problematic image was exported from Affinity Photo, and is attached as PoroSadE_128x_broken.png; the re-encoded, 'fixed' version is attached as PoroSadE_128x_reencode-working.png