You can also do it in Keynote. Just create hyperlinks between slides (which you can do with any shape, via the Inspector). Then export the presentation as a PDF, and the clickable image hyperlinks will be preserved in the PDF file. Then it's also very convenient for sharing with designers/developers/investors, etc.
Here's a really goofy and poorly-thought-out example as a proof of concept. It took about 10 minutes to make. http://goo.gl/AemLD (best viewed in Preview in single-page mode)
I do something similar on OSX with OmniGraffle. You can generate an HTML output that it's basically an image with links to other pages. It even works on PDFs.
