BB10 Does not currently support a URL for an ImageView. There was mentions during the beta stage that it was coming but it is no longer listed on the roadmap.
--edit: As per Kernald - this feature is still in the roadmap.
Javascript does not support direct downloading of files which means you will have to implement some c++ if you want to accomplish this.
The best example of this is from one of the official examples here: https://github.com/blackberry/Cascades-Samples/tree/master/imageloader
You should be able to copy some of the c++ classes and implement it into your app without too much difficulty, even if your knowledge of c++ is weak.