You can add text captions to any of the pages in your coloring book. Simply include the desired text within the <page> entry. E.g...
<page source="chicks.gif">Look at those cute chicks!</page>
You can even add some HTML to the text. When adding any HTML it is best to include the caption inside a CDATA section (this isn't complicated, just copy the example)...
<page source="chicks.gif"><![CDATA[<font face="Times New Roman" size="20" color="#0000ff">Eggs</font> are laid in a
<i>clutch</i> of around 12 eggs. <a href="http://en.wikipedia.org/wiki/Incubation">http://en.wikipedia.org/wiki/Incubation" target="_blank">Incubation
takes around 3 weeks.
Note that any line-breaks in your caption will become line-breaks when it is displayed, so be sure to only use them where you want them to appear.
For more details read about the Available HTML tags.