2
All these variables point to something specific.
To use <{$xoops_url}> which points to the root directory of the site, it would be something like:
http://<{$xoops_url}>/filename
or
http://<{$xoops_url}>/subdirectory/filename
To use <{$xoops_imageurl}>,which points to the image directory in the root of the site, you would:
http://<{$xoops_imageurl}>/filename
or
http://<{$xoops_imageurl}>/subdirectory/filename
The image directory by default is for images, other types of files may end up there but it is still the image folder, therefore the imageurl logic.
HTH