For those albums that had the albumart stored in their folders (e.g. because you had to access them with WMP which needs the folder.jpg file to display the albumart)
You can read that file with the help of an action (Press Alt-5, then create a new one of the type import album art).
The other albums may be retrieved if you have already entered artist and album as this the data that makes an album unique. Still, I think you have to go through each album individually.
THere are some threads in this forum that deal with the import of albumart, e.g. Automate the albumart process?.
I do not know how you think that mp3tag should be able to find out, how to fill the tracks in a more or less amorphous collection of data. To find the tags in a database you have some kind of unique identifier, e.g. for CDs this is the CD id. Taking this CD id it then can be said what contents the labels of the first, second, third and so on track should have. If you have simply mp3s but no filled tags on your drive then there is hardly any data clue what to look for. mp3tag kind of had to "listen" to the track - which it does not.
Can you tell us something about your filenames and directory structures and whether these contain information about e.g. artist, band, album, track, title? If so, the function "filename - tag" might rescue a fair number of tracks from data-wise ignorance.