Oh No Sir, Google can view images since a long time. There was a big testing and skilling of google for years to understand what a object on a image is. If you bring out a image about a grapefruit and alt them apple google would judge this in the future.
The Base idea of indexing images had google after serveral problems with the indexing of Flash Contents. There was for 2 or 3 Years a open Beta with people could learn google what objects are on a image and that was a more complex algoryhtmus than recaptcha.
Google is smarter than you think. It can see objects, text and people on images. I would be realy careful what you do with ALT Tags.