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.
Make sure you have configured your galleries to allow free downloads.