The homepage on my sight looks terrific, but has very limited text content on it. It's almost entirely images. I don't want to spoil my layout by adding visible text, but it occurred to me that I could hide some text behind an image, or make the font colour the same as the background colour, in order to give the search engines something to index and help my site ranking. Will I be penalised for this?
The practice of hiding text content from users is known as cloaking, and will lead to your site being banned and removed from the search engines.
The search engine spider should always have the same view of your website that any user would see. If you've designed your site such that it's very heavy on images, but light on indexable text content, then you didn't considered the search engines when you designed the website and you have a problem that you'll need to address visibly. Your site isn't accessible.
Never be tempted to hide any content in order to achieve better search engine results. Any attempt to manipulate the search results will result in severe penalties. Think more carefully about both your users and the search engines when designing and planning your website.