eBid uses an image with a maximum of 600 pixels along the longest side - with 600 x 600 square being the optimum. You can upload larger images, but eBid will resize them down to the above - so anything greater than that is a waste. Larger files can take ages for eBid to process, the resizing routine may not be the best (you may do a better job with a standalone program - irfanview does batch resampling of images).
Self hosted images used in descriptions can be any size, though should be kept below 980 pixels wide - as that is the eBid page width.