Sounds like the path to the page in the iframe is different than the path to the page containing the iframe. If this is the problem, then the paths to the images on the page in the iframe should probably be absolute.
Also, and I'm not sure if this is an issue here, but your iframe must have a name. This is not well documented on the lytebox site, but is required for this to work. However, since the lightbox launches, just doesn't show the image, it sounds like you already have this covered.
There could be other problems in addition to or instead of these, I'd have to see it in action to be sure. If you want more help:
Please post a link to the page on your site that contains the problematic code so we can check it out.
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