optimize images on google

One tip (more tips will come later) is to use the "Alt text" and "Title tag"
 
This is one of the most important part of image optimization and you should write a keyword or a phrase which is accurate and relevant to the image and to your webpage. The "alt text" should be inserted in the code for your image file as well as the title.
For example: <img src="filename.gif" alt="Write your alt text here" title="This is  the title of your image"/>
 
Actually, the alt text will be used by the browser (and therefore by the search engine) to understand the content of your webpage and to know why you used an image.
Also vocal browsers (for disabled people) will read the alt text of your image
Basically, the alt text will be displayed if your image cannot be shown, and the title tag will appear when your mouse comes over the image.