How to make a free low res download, paid high res download
These steps will let you setup a scenario where you can allow users to download a low resolution version of an image for free, but sell the high resolution file for any price you want.
Create a high resolution digital download product (more on creating digital download products here)
Create a low resolution digital download product. Do not set a price (this will prevent it from being available for sale). Also check the option to include it in any free downloads.