The problem with the cover/fanart preview in the html card display I initially thought is related to the size of the jpg image file located in the Databases/xx_Cover directory. I noticed that cover images with a size above 800KB or 900KB would not display properly.
After some experiments I'm not sure if the problem is the size of the image file or there is a certain type of jpg format image that is causing the display problem. I used the 'Paint' program that came with Windows 7 to resave an image file that would not display in XMM.
Original 2089 x 3209 (W x H) 1011KB does not display Resized 1820 x 2900 (W x H) 1820KB does display
The resized image has smaller screen dimensions but the file size is bigger.