I have the same question. I'm currently using an external program (Mp3tag) to add album art, but I'd like to do it from within foobar2000.
I use foo_run services to use metamp3.exe to embed art for mp3 files. In my examples, the "folder.jpg" needs to exist in the folder already. Here is my code for mp3 files:
Metamp3 - embed Front:
"C:\Program Files\foobar2000\tools\metamp3.exe" --pict {3}"$replace(%_path%,%_filename_ext%,)folder.jpg" "$replace(%path%,%filename_ext%,*.mp3)"
Slightly OT, but only due to file types, you can use NeroAacTag.exe to embed or remove files:
NeroAacTag.exe_AddFrontCover:
"C:\Program Files\foobar2000\tools\NeroAacTag.exe" "$replace(%path%,%filename_ext%,*.m4a)" -add-cover:front:"$replace(%path%,%filename_ext%,folder.jpg)"
NeroAacTag.exe_AddBackCover:
"C:\Program Files\foobar2000\tools\NeroAacTag.exe" "$replace(%path%,%filename_ext%,*.m4a)" -add-cover:back:"$replace(%path%,%filename_ext%,Back.jpg)"
NeroAacTag.exe_REMOVECover:
"C:\Program Files\foobar2000\tools\NeroAacTag.exe" "$replace(%path%,%filename_ext%,*.m4a)" -remove-cover:front"
Hope that helps.