FEATURE: Allow some customisation of thumbnail

[mellm0]

I have not edited the CSS yet, so it won't fit well for items that use different width and height, but you can now use PaddedImage etc.

I didn't have much luck with my quick look at the CSS...

I added a second commit, using getFormattedImage as well, since sometimes only a generateImageFormat method is provided (because that's how it is in the docs)