Each Room type is allowed 1 image. In some cases, it is attractive to make a collage of images. If you are selling through the GDS, you are not allowed to use collages and the images must be (300 x 300 min and 480 x 480 max). If you are not selling through the GDS then it does not matter what size images are used.
Thumbnail Images - these are part of the room type images section. All thumbnails need to conform to (100 x 100 min and 200 x 200 max).
A special tip: is to make them all the same size. It will look best on the booking engine.