Set the scrolling on the iframe to "auto". Then it becomes a matter of ensuring that your content is no wider than the iframe. Figure on 16 pixels for the vertical scrollbar's width in IE and most Mozilla based browsers. Add another 4 pixels if using a frameborder="1". Also setting marginWidth="0" in the iframe tag can help, otherwise margins will need to be taken into account too. For example, in IE6 showing a 590x1000 pixel image through an iframe, I used:
HTML Code:
<iframe src="../layout.png" width="606" height="777" scrolling="auto" frameborder="0" marginWidth="0"></iframe>
It had a vertical but no horizontal scrollbar. Showing images in Mozilla presents an additional problem, they often need to be on a page so as not to be resized by the browser but, since you are dealing with pages, that should not be a problem. Once I overrode the resizing in Mozilla, the results were comparable to IE. There is an easier IE only way of dealing with this but, you wanted cross browser, I take it.
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