At this point more details will likely be needed. However, from what you've described my initial thoughts are these:
-If you just created the Windows 7 image, it simply may not have replicated yet to other Package Servers. Try again later.
-If the Windows 7 and Windows 8 image are both being downloaded from the same Package Server:
-The IIS/UNC/DB path to the Windows 7 image could be incorrect.
-Does the Win7 image exist on the drive where the console says it is?
-It is possible that these two images exist on different servers and one is configured correctly while the other is not.
To tell you more, please gather the logs from the failing Windows 7 image deployment while the PC is in WinPE from:
X:\Program Files\Symantec\Deployment\Logs (zip up all logs here)
Then gather the logs from the succeeding Windows 8 image in Production under the following location:
C:\Program Files\Altiris\Altiris Agent\Agents\Deployment\PreOS_logs...\Dated folders
Attach the files here and I'd be happy to take a look.