Are you adding the images as the downloadable File in the Download or as an image associated with the Download.
If it is an image associated with the download then a smaller 'thumbnail' version will be displayed when looking at the Download in the front end. Clicking on the image presents a larger view.
You can set the size of the 'thumbnail' in Config - Pictures - in the Thumbnails Settings section.
You can also set the thumbnail to be created automatically from the uploaded file, if it is an image type file, in option 'Create Thumbnails from uploaded pics?'I will also send you a private message (PM)