I have to host images for the freehosted blogs on one of my own hosted domains, so it's going to be the same thing really if you use the domain with the BO installation to hold your images.

1. You will need to add into your htaccess on the BO domain all of your domains that you're allowing to hotlink

2. From an organization standpoint, you'll probably find using one domain for the images a lot cleaner approach, plus you know where to go looking if that evil red x starts showing up!
$0.02