There is nothing wrong with the images in your feed. If you view the feed in your web browser you will not see them because it is not a web page it is an xml feed and they are not normal image tags.
However if you right click on the page and view the xml source you will see that it contains the correct image tags as it should, for example
<g:image_link>http://www.dpieces.fr/images/stories/virtuemart/product/c00076445.jpg</g:image_link>
If you check the image url you will see that the image loads as it should.
Your robots.txt file is fine, but, if Google has recently asked you to change it so that they can crawl your images folder, you need to be aware that it takes them some time to do this, it can take up to a week, there is no alternative but to wait unfortunately. There is no other problem with your images.