the best method for google indexing text content in images?

I have a webpage where I put 1 image once in a while, this is just like xkcd.com I would like to know how to let google know the text in my images.

My approach is to put the text in alt html attribute, like this:

<img 
src="http://myapokalips.com/pu开发者_高级运维blic/cartoons/021_Robot_Tattoo.png"
alt="RETARD - aw, that's a sick tattoo! - yea, bro, it means 'strength'" 
/>

is that right? or should I put the text in some random metatag?

thank you in advance.

Just do it like xkcd.com is doing.