Unless the image dimensions are specified by the marketplace creator, there is no such thing as "actual size". Roll20 uses an arbitrary 70 pixels per grid unit (most commonly 5 feet, but by no means universal). A map creator may have decided that 5 feet is represented by 72 pixels, 96 pixels, 140 pixels, 300 pixels, or a number just grabbed out of the air. There is a long term project to have marketplace creators tag older images with their intended display size, but I do not know where it stands. Roll20 could have every image come in at 1 pixel equals 1 pixel upon drop, but most tokens are double resolution or more so they look nice when zoomed in. Having images drop at 1:1 would mean that all tokens would come in at double size or more. Hence the need to tag intended display by gird unit. This has not been achieved yet, but I have been told by many folks who have reason to know that it is a work in progress.