The eCommerce functionality is powered by "Easy Digital Downloads". Check out the full EDD documentation here: http://docs.easydigitaldownloads.com/
Creating "Download" Posts
To add a new download post, click on Downloads > Add New. To add the images, make sure to upload a "Featured Image" for each post. You can apply watermarks on these images and then simply upload a zipped file into the "Download Files" area, if you wish to block users from accessing your full images before the purchase.
Default Image Sizes
The default image width for the index is 800px, while the default size for the single download page is 900px. Both of these image sizes do not have a limited height, so you can have both landscape and portrait images on your index. You can however change any of these sizes by editing the functions.php file.
Regenerating image sizes after adjusting settings
After changing these settings you may need to regenerate your thumbnails. You can do this with this plugin: http://wordpress.org/extend/plugins/force-regenerate-thumbnails/
Download Pricing Options
You can easily add multiple product pricing depending on custom attribute (e.g. image size in pixels).
eCommerce Options (Columns, Posts Per Page)
You can adjust the eCommerce archive pages options under Appearance > Customize > Downloads