Using the new feature type image, create features:
tile (AM)
standard (or basic)
thumbnail
And maybe a medium size between standard and thumbnail — or wait until it become required — your choice.
I think that the dimensions should form a bounding box that various shapes could fit within.
I'm letting you decide on precise name and image size.
PS. Ultimately we should have landscape and portrait versions of each image feature — but work on landscape first.