Hi,
I just wanted to dispel a few myths going on here...
- ALT tags are NOT to 'Tell Google' what the image is about, they are there for people who use screen readers to enrich their use of the internet.
- Screen readers can be set up to read out alt tags, file names or title attributes - depending on the user's preference and presumably on the site they are looking at
- Google can and does read image file names: http://blog.hubspot.com/blog/tabid/6307/bid/32567/A-Marketer-s-Guide-to-Optimizing-Images-for-Google-Search.aspx
If you pick relevant images for your pages then you probably will be able to get keywords into both the alt tag and the file name. I don't think this is spammy! It's accurate.
What would be spammy, and this has already been covered by Issac, is if you repeat the same alt text on the same page - vary it! - 'Blue lego bricks' etc. I personally would avoid saying stuff like 'cheap', 'buy' etc UNLESS the image has that text within it (say a picture of lego with a headline within the image saying 'cheap lego') in which case you are doing exactly what the alt tag is there for - enabling screen reader users to experience as similar experience to us sighted folk as possible.
Sorry for the rant, but it annoys me that something that is meant to help people who are (to my mind) at a disadvantage is abused for SEO purposes!
Best wishes,
Amelia
**EDIT: I am well aware that Google and other search engines use alt tags for rankings. What I wanted the OP to be mindful of is that alt tags were not invented 'to tell google what an image is about'. And to point out their value and use for people using screen readers. Most people do not fully understand how blind users navigate the inter-webs, me included. I just think if I can do something to make it easier for them then I will. If you'd prefer to write alt tags purely for the search engines, that is entirely your prerogative! **