I've opened a bug for this: https://www.ventismedia.com/mantis/view.php?id=18050

That only allows one image at a time to be added to the tracks. MM4 allowed me to add all the images at the same time.

You need to use Browse on the Image search window to browse the local PC for images.

When I try to add artwork to existing library entries using the 'improved' MM5 version of the "Artwork" tab in "edit properties for multiple files", "Lookup image" returns a huge number of tiny icons that are impossible to discern.

MM4 used to show just the graphics (and accompanying text) found in the folder containing the multiple files.

There many great improvements in MM5, but this is a step backward in functionality from MM4.