Above already said about use alt tag and title tag. In case you do not familiar with HTML I will provide a sample code <img src="http://www.domain.com/images/images-name.jpg" alt="keyword" title="keyword"> Code (markup):
adding images to your site maps maybe you should read my blog about adding images to your sitemaps, here: http://edithhadiansyah.com/adding-images-to-your-sitemaps/
First of all, the image that you want to optimize should have a name including the keyword for which you want to promote it. example: companyname_contact.jpg Secondly, always use the alt and title attribute as Sxperm explained. Using height and width attributes in the img tag will help the google to search if a user specifies the height and width in his search query. Best of Luck
add alt tag and if you new seo then you will have to read google guide lines after that you can optimize your website properly..
if you are using wordpress there's a good plugin called SEO Images, that automatically adds your post title to your image title ...