Podcast artwork shows in Apple Podcasts and iTunes but not on iPhone
One solution is to embed the artwork into each audio file. Command-line solutions include lame and ffmpeg, for example:
lame --ti /path/to/file.jpg audio.mp3
ffmpeg -i input.mp3 -i cover.jpg -map_metadata 0 -map 0 -map 1 output.mp3

See more at: https://unix.stackexchange.com/questions/84915/add-album-art-cover-to-mp3-ogg-file-from-command-line-in-batch-mode
You can also download dedicated software for this purpose, though I did not try it.
Either way, if you handle the low-level of podcast distribution, remember to update the length attribute to reflect the now larger file, or else the duration may be computed from the file size and display the wrong number.