are there simple alternative ways to verify where i need to upgrade/upsize album art by viewing album art dimensions whilst viewing music files.
The best way I've found is by using the tag image display we've been talking about. That doesn't allow me to filter those needing replacing, but I need to see the image to decide what's needed anyway. I size the window so it will show my target 600 pixel size fully. It's then obvious when a cover is significantly smaller, and it's otherwise large enough to assess the quality. An image sized to 600 pixels isn't necessarily of good quality. A tooltip also indicates the size of the image file, which sometimes helps.
Although I can only check one album at a time, I can see instantly whether a replacement is necessary. I use a nested Artist\Album pane to select an album, then just press
Down to move through them. This makes it easy to spot covers that might be okay on their own, but which should be upgraded to match the quality of other covers for the same artist. I've already replaced all the terrible covers. Now I'm looking for consistency and incremental improvements. I'm quite sure looking at dimension and file size numbers wouldn't help much.
The attached screen shot illustrates this. The existing cover is 500 pixels—obvious from the fact it's not filling the 600 pixel space provided. I'm not necessarily going to replace this because it's not 600 pixels, but I can see the quality is poor, and the tooltip confirms it's small file. So I find a replacement using a search link. Having the image display open makes it easy to find the right cover and see the difference in quality.
As I've mentioned, I see the same issue you do. After replacing the covers using
Add from file, I have to do also
Rebuild thumbnail and
Refresh to see the replacement in the image display. If I don't do both and visually confirm the replacement before moving on to the next album, there seems to be a significant likelihood the thumbnails won't be fully updated. But now that I know the extra step is necessary, I don't consider it a big issue.