(UI) Possibility to select a preview image for an item

[sebastfr]

case-study: see http://eprints.gla.ac.uk/39513/ and its PDF. There's a coversheet which is rendered as the preview, not ideal esp when you do see the actual cover of the book (which is colourful and rather pretty).

It'd be nice to be able to either select a page from the PDF as the preview or to upload a cover for the publication. Covers shouldn't be shown as part of the item's Documents.

Also the ("small") preview on the summary page is really small and leaves a fair amount of white space next to it. See e.g. on Amazon/books how the images/data are presented. Their preview is 160x160 against 60ish on EPrints.