Hi Amy and welcome to the RH community.
That is the way that image maps work. If you are using an
image map, the linked topic must reside inside the same project.
You could link to an external file by clicking the down arrow to
the left of the "Link To" field and selecting file, but this does
not allow you to display in a popup. Also if you are using CHM
files, the selected file would have to be the CHM which would open
with the default topic. If you were using Webhelp you could select
the individual HTM file.