You don't need a custom image for what you're describing (and you couldn't create one if you needed to anyway). The emulator image contains only the OS functionality itself. The skin, buttons and screen size are all configured in an XML file.
As an example, install the WinMo 5.0 SDK and then navigate to here:
C:\Program Files\Windows Mobile 5.0 SDK R2\PocketPC\Deviceemulation\Pocket_pc
You'll find Pocket_PC.xml. In it is describes things like the skin images (as well as button actions):
<view
titleBar ="Pocket PC - WM 5.0"
displayPosX="55"
displayPosY="67"
displayWidth="240"
displayHeight="320"
displayDepth="16"
mappingImage="pocket_pc_emulator_mask.png"
normalImage="pocket_pc_emulator_up.png"
downImage= "pocket_pc_emulator_down.png">
You'll see that the skins are in separate folders at the same level as the 0409 folder, which contains the actual CE image.
Browse around and you'll see the same layout for Smartphone, and in fact the WinMo 6.0 SDK is the same way (not coincidentally).
Now head over to Studio and look in Tools->Options->Device Tools->Devices. Select any given emulator and click "Properties" and then "Emulator Options". Here you'll find another way to adjust display settings and skins. You can copy any existing emulator configuration by clicking "Save As" and then adjust it to match what you're after.