When you download your copy of your products using the Product Export tool, you'll have a choice of an XML file or an Excel file.
If you choose Excel, the following are the columns in the sheet:
Thumb (image url)*
Medium (image url)*
Large (image url)*
* Time stamp: if any of the variables with an asterisk change, the time stamp on the item will be updated. The alerts your software that a recent change has been made and you'll likely wish to refresh the product's data.