To remove the Windows default Image Viewer, first:

Click Start Menu
Select Run menu
Type "cmd", (for Windows Me, type "command")
Then type "regsvr32 /u shimgvw.dll" to unregister this dll. This will stop it from previewing any picture that it support, e.g. JPEG, Bitmap, GIF....
* Before perform this tweak, make sure that you have the alternative Image Viewer installed in you windows e.g. ACDsee, FireGraphics... because once you do this tweak without that application, you can't open and view your image anymore! So, to undo it, type "regsvr32 shimgvw.dll" in command prompt.