preview opens blank jpg over vnc

Hello and Happy Thanksgiving to everyone.

I am using VNC to access my iMac from a Windows 7 machine. When I open a .jpg in Preview, remotely, I get a blank Preview window (except for the file name at the top and the various icons, i.e., "Previous," "Next," etc.). This happens when the iMac's display is showing the Login Window. Does anyone have any idea why this is happening and if there is a fix for it?

This happens both with RealVNC Server in User Mode and Vine Server, and with various clients on the PC. When you open the file on the iMac itself, the image displays correctly. And, when the iMac display is showing my user screen as it displays remotely, the jpeg is rendered correctly; it is when the iMac is showing the Login Window that the problem occurs.

On the iMac, I am running Mac OS X v10.6.8.

I tried deleting com.apple.Preview.plist, but it didn't seem to solve the problem.

Thanks in advance,

Mark