Go to Dashboard, Ecommerce and click Generate Thumbnails. 

• "Generate all thumbnails overwriting existing ones" option creates new thumbnails and links them to each product. It does not remove existing image files
• If you click "Generate all thumbnails overwriting existing ones" you can enable "Remove old thumbnails" option and remove unused image files
• "Max width and height" options set a rectangle where every thumbnail must fit.
• If you are creating thumbnails from JPEG you can set the JPEG compression you want, between 1 and 100. The higher, the better but the bigger. 
• All new thumbnails will be grouped by the "automaticallyCreated" file set
• Once you set all options you can click "Generate thumbnails" and go.
• You can open a new browser and go to the Logs screen to display all operations running.