Allow image upload of pre-rendered thumbnails

[dserhienko]

In the case of very large models, such as dragons or terrain buildings, the model often only exists as a collection of smaller parts

Individual thumbnails of those parts isn’t greatlt helpful in identifying them

It would be very useful if we could choose a pre-rendered image file to upload that the program would then see to use as the default image for display in the gallery.

Clicking on the item could then show the individual item rendered in some manner.

[dserhienko]

My other use case, unmentioned, would be to upload multiple images for a given model/compositional model. This would be especially helpful in cases like the recent Tartessos Kickstarter, which is a 'build-your-own' model set, where you mix and match heads and arms and bodies etc, and there is no base. That would also apply for the Ship Works kickstarter.