I could have sworn there was a thread where you could post items that needed an art update (to fit into items in the same series, for example). Did I dream it or where is it?

it's a subforum in Site Feedback. Subforums are above the main forum thread listings, and you were probably looking down instead of up. It's called Revamp Feedback

Also there's a place for art errors in Problems & Bugs if it's part of a set that is wrong somehow. Sometimes they will 'correct' it instead of revamping the entire set.

thanks

you are welcome