Use your "Reference Manager" tool: start/all programs/autodesk/autocad 2008/reference manager (I think all installs of autocad come with the tool, and the path to the manager might be slightly different on your computer). Open the manager, drag and drop the .dwg file that has the image that you would like to repath, scroll through all the referenced files and find the image and click on it, then click on the "edit selected paths" button at the top of the screen. Then you can type in the new path, or copy and paste it. Once that is done, click the "apply changes" button and the image should now be repathed to the correct location. I hope this helps!
Brian