We're monitoring this post to see how many folks are interested in this functionality, which falls firmly in the "nice to have" realm and would require us to pull a developer off of more urgent or impactful issues. That's certainly not to say we would never do it, as it's a great feature request, just relatively low priority.
I did move it to a more prominent place internally where it will get attention eventually.
In the meantime, The image on the donor wall and the back end donor profile supports Gravatar, and so if folks have a Gravatar set up for the email address used in the donation, that image will populate there.
Vote and comment here! We do listen to our users and prioritize accordingly.