Importing XML using feeds API
It depends somewhat on your use case. For example are you scraping the images off a web page or are you using a direct image file URL.

If you are scraping from a web page you can use the feeds image grabber module.

That module provides additional settings where you can configure a CSS selector for the page that is in your XML file. 

The module will then request that page and scrape the image from it based on the selector you configured then add the image to the imported item.