A search for image maps in the wordpress.com Support doc or the forums doesn't offer any help regarding using image maps in a wordpress.com blog: Search Results for image map « Support — WordPress.com
The area tag, needed for image maps is listed under allowed HTML codes for wordpress.com.
WordPress.com allows the following HTML code in your posts, pages, and widgets:
address, a, abbr, acronym, area, b, big, blockquote, br, caption, cite, class, code, col, del, dd, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, img, ins, kbd, li, map, ol, p, pre, q, s, span, strike, strong, sub, sup, table, tbody, td, tfoot, tr, tt, ul, var
Image maps are handy when it comes to providing links within an image itself.
If you still don't get it, maybe seeing an example will help.
This link contains defiinitions of image maps, how you can create your own image map and also an example of a working image map in a wordpress.com blog:
How to add image maps in a wordpress.com blog
Enjoy!
psst: Image maps seem to be new around wordpress.com. When I (or other users) tried image maps years ago, wordpress.com didn't accept the needed codes - a search for image maps in the forums reveals that.