Thanks for your reply. The two computers are in the same LAN, they can visit each other in the netowrk. There is no database connection.
I just found out the reason why the image server can't read data. Because the data folder on B is mapped as a drive Z:\ on A, and when I added the data as service, I used the mapped path, like Z:\images\... , But when I use the absolut path, like \\B\C:\images\... the image serve can load the data correctly.
Are you publishing the image services from an Image Service Definition (ISCDef)? When you author a service using the Image Service Definition editor you are connected to the share using your account. When you publish the ISCDef the access it done by the Service Provider that runs as a service. By default the Service Provider runs as User SYSTEM. SYSTEM will not have by default access to the share. You either need to change the share access to allow SYSTEM read access or change the user of the Service Provider process to a user that has a log in. (use Services.msc). The reason you can see the overviews is the the overviews would be by default on the local machine the SYSTEM has access to.