When adding images to your pages, in XSitePro, you will always be presented the Alt-text dialog:
It is strongly advised that you get into the habit of quickly entering an apt description for the image in question, using this feature, since this will save you time farther down the line.
In case you may have missed some (and to be sure that the alt text you have used is appropriately descriptive) there is a handy table of information in the accessibility details screen dedicated to your images, and their associated Alt-text.
Alt text is not only useful for search engines to determine the nature of your page content, they also serve a practical use for people browsing your site using some kind of screen-reader. Images can effectively be 'read' by a visually impaired visitor, but only if you have taken a moment to enter appropriately-worded tags.
Looking down the table you will see two columns:
Scrolling through this list you can perform a quick check to make sure your images all have Alt-tags where appropriate (more on which at the bottom of this page).
Sometimes it is better to keep images 'silent' in your page (i.e. to have no Alt-tag for the image).
This is generally practised when an image plays no part in the informational content of your site (for example, component pieces of a design element, spacer graphics, etc.).
Where you have used these unimportant images it is often better to leave these images with no alt-text so that they remain 'silent' when viewed using a screen-reader.