In Virtuemart 1.1.xx picture adding functionality by default is not possible. This is why we had to hack virtuemart to add this functionality. There were some PHP files that are in the Administrator folder which comes with Theme Installation file. Are you sure you did copy-paste them?
Please follow this tutorial:
www.flexiblewebdesign.com/blog/show-thumbnails-in-cart-modification/
We did the same thing in our Theme but they were supposed to be pre-sete. I don't know what went wrong and doesn't work in your case.