"Could not be used because the original file could not be found" with no access to original file.

Leaving the artwork issue to one side for now, let's concentrate on the file can't be found messages as that's the more serious problem.

It sounds to me as though you've gone into Windows Explorer (the file manager) and deleted the songs from there, leaving the entry for the song in your iTunes Library. Hence, when you attempt to make use of a song listed in your Library, either to play it or to synchronise your iPod with your iTunes Library, iTunes can't find the file, because you've deleted it.

Bear in mind that you do not "transfer" a song to your iPod from your iTunes Library, instead you copy it, leaving the original file where it was.

You should not be deleting the file (for a song) on your computer simply because you've added it to your iPod. An iPod is not a backup - your iTunes Library is the backup for when your iPod gets lost, broken or stolen, or simply requires a wipe clean and start again (a Restore). Even then, you should be backing up the files (the songs) that are on your computer to somewhere else (an external drive for example).

If you don't have enough storage room for the music on your computer, you need to buy an extra drive for additional storage. My recently purchased desktop has very little built-in storage (that appears to be the modern way, so I purchase d an additional internal drive for things such as music and then I back that drive up to an external drive.

So the first thing to do is go into your computer's recycle bin and restore any deleted music files that you can find. For any songs that are no longer in there you will have to find an alternative solution. If they're from CD, re-copy the CD into your Library. If they're online purchases, go back to that store and download them again, in case they ever become unavailable.

Then make a back up.