I've added a lot of image files mostly Covers for Movies and Covers (Images) for People. Adding the image works fine and I can change its position and delete other images without problems.
However, the creation of resized thumbnail images (which are displayed in the HTML Export) isn't automatic.
Thumbnail images are only created when you Select 'Update from Web' or 'Re-Update' and only if the source website has an image on file.
Adding an image when no image exists: A new image will be added but the image displayed in detail view (movie_bigcover.htm) (hover over actor image) will be full-sized and not rescaled to a thumnail image height of 240 pixels.
Changing an image when there is an existing image: A new image will be added but the image displayed in detail view (movie_bigcover.htm) (hover over actor image) will be the original image and not the one you just added. It won't matter if you repositioned or deleted the original image.
You can fix this by Pressing 'Update from Web' or 'Re-Update' but then you have to go back and delete the original image again but this will only work if the source website has an image.
Another solution which is rather slow is to use Tools - Cover/Picture - Rebuild Thumbnails (for movie covers) Tools - Cover/Picture - Rebuild People Thumbnails (for people images)
To fix this problem: XMM should be reprogrammed to automatically rebuild thumbnail images when closing the cover manager if: 1) a new image is added for the first time or 2) the first image is deleted or 3) another image is repositioned to be the first (primary) image
(the scripts to do this already exist and are used in 'Update from Web' or 'Re-Update')