OK, I feel a little foolish. Just a quick note in case you made the same mistake I did. When you are creating a custom ad in Mobclix (I believe it is the same for iPhone and iPad, but I did this for Android) the process has a little trick.
Unlike Admob text ads, or Adsense ads, the “Banner URL” does not refer to the URL that will be displayed to the viewer (either as part of the ad or when the graphic can’t be shown). Rather, it is the location of the graphic resource that you want to use. You can self-host it or put it on the web in some other manner.
In other words, rather than uploading your image, you just tell them where to find it. I don’t know if that causes performance issues or if they cache the ad and serve it.
Either way, I resolved my frustration with the “broken” Mobclix custom ads.