To search engine spiders, an image is just a meaningless data file. This means you can help search engines by using distinct filenames for any image you use. Moreover, when inserting an image in the Joomla article editor, add a specific alternate text.
This Alternate Text (or alt text) is the short description that is shown when the image itself isn't displayed for some reason. You may also want to add a similar description to the Title field (under the Advanced tab). This title text pops up when the visitor's mouse pointer hovers over the image:
Tip
More on SEO-aware writing
Of course, creating great content isn't a Joomla-specific art. If you want to read more on SEO-aware writing, there are many resources on the Web. To get a clear overview of the basics, download Google's Search Engine Optimization Starter Guide from www.google.com. Sites such as Copyblogger.com offer numerous tips on writing effective copy for the Web.