Assign all subnets to sites. Make sure each site has a site server assigned which contains the resources needed by computers in that site. The typical configuration would be Site A, Site B, Site C, Site D, Site HQ. Site HQ has the NS and a site server running task, package, and deployment services/handlers. You want to image at sites A and B, but not C and D; therefore, you put task, package, and deployment services/handlers at A and B but only package service on C and D (since you don't need to image there, you don't need the deployment handlers, and therefore do not need task, either). Finally, your other geographical sites W, X, Y, and Z are small over fast links, so although you assigned the subnets for these sites to the sites W, X, Y, and Z in Altiris site maintenance, you didn't assign local site servers. Instead, you assigned them to the HQ site server. W/X/Y/Z have fast links and will go across the WAN to the site server at the HQ data center for package data.
If your .gho files are replicating as part of standard Deployment Solution stuff, your .gho files will be present at Site HQ, A, and B. They will not be present at C, D, W, X, Y, or Z, so you cannot image there.