The search engines can’t really see anything to index when it comes to images. When they spider your page, they’ll see html which is made up of loads of text they can make sense of. For images it is a little bit different.
To make this easier for google and the rest of the main search engines, you need to add a bit of text to the image so they can see what it is about. This tag, called the image alt tag can be placed in your CMS or admin area. For example on this site (via WordPress) they make it very easy for you to do this whenever you add an image. You just add a title, caption and an alt tag as and when you wish to do so.
Now the alt tag isn’t as important as some other tags on the page, including the title tag and header (h1 tag for example) but it is worth adding and will allow the search engines to identify what the images are about on your pages. So, just add a few keywords on what the image is about and it will then have that text tag associated with it. The image may also show up if someone does an ‘image search’ and types in that keyword. You have probably seen this in google where you can find an image on almost any subject. A lot of site owners report very decent referrals from image searches alone, but this can sometimes mean that someone just wants to copy the image from your site. Have a look at your analytics package to find out how long these people are staying on your site and how many pages they are looking at – this is a clear indication of quality.
Adding this tag will also add to the keyword density of your page to a certain extent so it is well worth adding. It is just another instance where the search engines will find one of the main keywords on your page and does help.
The search engines are getting more sophisticated when it comes to images. They are now developing systems to work out what your images are about and this includes the ability to work out a percentage of colour density. This allows them to see if any images are of human beings (because of skin tone colours in your image) and even if they are naked! It is all very complicated so just keep it simple and add these tags each time. Don’t add too many keywords though – it is better to describe what the image is about with the keyword it in, so “Dog training for our German shepherd” is better than just “dog training.” They can work out if you are just doing it to place your main keywords so add some variety to it!