Since people asked, here’s how I make Mp3tag embed album art in the mp3 tags and convert Replay Gain to Apple’s Sound Check.
Both require a new set of actions. If you go to Convert > Actions, you’ll bring up the action list and can create a new action group. For album art, make a new action group. Add the “Import Cover from File” action to the group. Enter the name of the file that has the cover art in the next pop-up window. For me, that was “cover.jpg”.
For converting Replay Gain to Sound Check, create a new action group and add two “Format Value” actions. For the first, set the field to “COMMENT ITUNNORM” and the format string to “$if($eql(%_extension%,mp3),$rg2sc(%REPLAYGAIN_ALBUM_GAIN%),)”. For the second, set the field to “ITUNNORM” and the format string to “$if($eql(%_extension%,m4a),$rg2sc(%REPLAYGAIN_ALBUM_GAIN%),)”.