As far as I could tell (looking at generated output and at the source code), the :image "label" (as noted in the documentation and source) is not used. From the documentation, I had expected it to result in an anchor being placed in the document, so that it could be referenced (via link) from elsewhere within the document.
To work around this, I tried to manually create an anchor just before the :image and reference that achor, but instead of being placed just before the image (the documentation states that the anchor would be placed "at the beginning of the next paragraph or image"), it was placed after the image, at the start of the following paragraph.
Furthermore, the documentation (helpsetmaker.pdf) states that the :image parameters are "a name, a sequence number, and a label", while (based on the examples) I would describe them instead as "a filename, caption, and label".
Finally, the examples in the PDF documentation also imply that the "Figure 46:" part of the caption is generated by HelpSetMaker (which would be nice!), but alas it must be hard-coded as part of the caption (I noticed that the "online" -- i.e. built-in -- help examples do not include the "Figure XX:" part of the caption in the sample output).
z3gator@gmail.com
P.S. I do very much appreciate the work that has been put into this very useful product -- by reporting this, I'm just trying to provide some (hopefully) valuable feedback.