Filter out non-standard cover images

[thusoy]

Cover images that are not close to 'normal' image formats for cover images should not be added to the database, since these most likely are not genuine cover images, but probably images of box sets or actors or similar.

Marvin can probably download the images he discovers, check the dimensions and only add it to the database if it passes the test. Images should be deleted once this is done to avoid filling up the server.