a few thoughts/questions: First, I had been wondering if image alt tags are searchable content the same way that page text is. Experimenting with my own page, I learned that the answer is yes. I hid keywords in alt tags and google found them and displayed them below my page title on SERP's. It then hit me that this presents an opportunity: if I have a page that targets a certain keyword phrase but it appears unnatural to show that phrase 3-4 times (or whatever the ideal number is) on my page text... then I can hide the phrase in a couple of alt tags on my page. Even banners and unrelated images can host the keywords. My question is: Is this considered "black hat"? Will google catch on to this trick and impose a penalty? And this leads me to a second question: Most of my site navigation is through clickable images and often several on the same page have the same keyword in their alt tag. For example, in the widget section of a website, there may be 10 navigational buttons that all contain the word "widget" in their alt tags. The intention is to pass on juice for the keyword "widget" to the destination pages but, as alt tags appear to be treated as page content, could this lead search engines to consider the original page with its 10 "widget" alt tags to be spamming for that keyword? (hope the question isn't too confusing!)
This is not blackhat to use alt tags, its actually encouraged because google will know what the picture is. Multiple links to one page will only be counted once by google. I would suggest NOT using images for navigation, it isnt nearly as effective as anchor text.