Suppose you want to display, next to each thumbnail, text to indicate that a full-size version of the graphic is but a click away; for example, Click to enlarge.
If you use extraction to create the thumbnails (see §27.8.3.2.3 Providing scaled thumbnails), you can do something like the following to put the text next to the image:
[ExtrReplace] Thumbfmt=<$thumbnail>
[thumbnail] <table border="0"><tr> <td><$_extrthumb></td> <td><p class="thumbtext">Click to enlarge</p></td> </tr></table>
Add an entry for p.thumbtext to your CSS, perhaps in [CSSStartMacro] if you have DITA2Go generate CSS each time. You could also give the table a class, and define its properties in the CSS file.