Note: if you want to read about optimizing images for performance, read the article I wrote about that. It is a long article ;)

Google can not 'see' images, but it can read all it's properties as set in HTML and can therefore use them to establish rankings for pages with images. In order to support search engines in doing so, you should supply images with relevant filenames and alternative descriptions.

First of all, you should start with images with relevant names. If you use an image about an Ipad, name it ipad.jpg (preferably only use lowercase) or something similar, as long as it contains the word Ipad. You can even use multiple words if you use dashes (preferred option) or underscores to connect them, like ipad-tablet.jpg.

Secondly, when inserting an image, you should enter an alt-attribute. This attribute is originally meant to support disabled readers with screen readers, but Google also actively uses it. You can easily add them with most editors. Both TinyMCE and JCE have options to enter alternate text. JCE auto-populates it with the filename, replacing underscores and dashes with spaces. So, if you have logical filenames, you can simply let JCE set a correct alt-text:

JCE even does this automatically by filling in the file name without extension, so if your file name is correct, you have your alt-tags set right very easily. Of course you can still write your own. Besides being good for SEO, it is also good for accessibility for disabled users that use screen readers. 

If you need a tool that checks your whole site for proper use of alt-text, I can recommend the SEO Powersuite Website Auditor. It lists how many issues there are, plus a full list of URL's with the issue plus the actual image affected:

When looking at images, also make sure to reduce image size as much as possible. I can recommend using the ImageRecycle extension for this, I use it here on Joomlaseo.com.