I was striving to do exactly the same inside the Chatter API - I needed to load image from feed data under "smallPhotoUrl". What I have found is, that at the end of URL active token has to be added.
This source introduced me to a concept: Accessing Chatter user pics
And I finally was able to find some clear information on how to access that token here: salesforce_platform_mobile_services.pdf page 262
So eventually I did this:
NSString *builtImageUrlWithToken = [NSString stringWithFormat:@"%@?oauth_token=%@",
phototoUrl, [SFAccountManager sharedInstance].credentials.accessToken];
NSURL *imageURL = [NSURL URLWithString:builtImageUrlWithToken];
NSData *imageData = [NSData dataWithContentsOfURL:imageURL];
I think it's relevant to this topic, as I came across it while looking for this particular solution. I am sure it could be useful for others as well.