こんなやり方もあるで。

static int cnt;
cnt = 0;

for(NSURL* url in URLArray){

NSURLRequest *request = [[NSURLRequest alloc] initWithURL:url];
[NSURLConnection sendAsynchronousRequest:request queue:[[NSOperationQueue alloc] init]
completionHandler:^(NSURLResponse *response, NSData *data, NSError *error) {

UIImage* serverImage = [UIImage imageWithData:data];
cnt++;
if (cnt == [URLArray count]) {
NSLog(@"finished");
}
}];
}