The reason why there is no image data returned is because you most likely do not have any url data inserted into the PhotoURL_Small and/or PhotoURL_Large field in the admin area of your store for your products. PhotoURL_Small and/or PhotoURL_Large fields are only populated from data in these fields. There is no built in Volusion method to have the image URL's appear within the XML output from a admin generated API call.
There is a way to return the image url in the XML output but it requires a different API approach and has nothing to do with the built in Volusion API admin page, all of which is beyond the scope of the question at hand.
Edit
Why not run a simple SQL query from the admin section? Home > Inventory > Import / Export
Assuming a "2t" image size. Change if you want a smaller or larger size.
SELECT
Products_Joined.ProductCode,
'Config_FullStoreURLConfig_ProductPhotosFolder/' + replace(Products_Joined.ProductCode,'/','-fslash-') + '-2T.jpg' AS Image_URL
FROM Products_Joined