It's easiest to use the append_graphics_path() function to add a new path to check for graphics (if you're not using the Preferences UI).
I did the following test:
1) Create a sample Docbook document, and save it in a temp dir.
2) Create an "images" subdirectory, and copy a generic graphic file into that subdirectory.
3) In the document, insert a <graphic fileref="sample.gif"/"> element, where "sample.gif" is the name of the test graphic.
4) At first the image doesn't display, i.e. I just get the missing graphic icon.
5) On the Arbortext command line, I execute the following: append_graphics_path("./images");
6) Now, position the caret after the graphic element, and from the menus choose "View->Reload graphic". The graphic should now appear.
This also works if you add "./images" to the graphics path via the Preferences UI. The key, maybe, is to make sure you choose "Reload Graphic" to see the change after you add the new graphics path. If you set it in Preferences, the result should be persistent, so that you can close the document, re-open it, and it should still find your graphics.