You need to be using ng-src (just an FYI).
<img ng-src="{{user.photo.pathRelative}}{{randomStr}}"/><br/>
And then you set random string after the upload.
$scope.randomStr = '?randomStr=' + new Date().getTime();
The downside is this will not work after refresh...
You could also combine the two if you want like @Tom mentioned below. I do not like doing this, bc you are modifying potentially an "object" that could be saved. For instance if you are using ng-resource and you modified pathRelative and then $save, it would result in the pathRelative being updated in the database.