How hard is it to create a screen-saver for Windows?

The reason I want to try is because Windows 7 does not support spanned desktop, so the pictures slideshow will not show dual-monitor images properly across both monitors.

Might have to get into the Windows API to get access to draw on the 2nd monitor? SDL could still handle the image loading/manipulation.

It's not that hard to create a screensaver for Windows, but creating a fully compliant SDL screensaver would probably take some hackery of SDL. The reason being that screen savers are required to execute certain code as part of existing windows, for example, for screensaver previews and settings. If you search msdn.microsoft.com you can get quite a few examples, including in C#, but if you want to start from scratch you need to handle three different command line arguments: /s to start the screensaver in full-screen mode, /c to show the configuration settings dialog box and /p XXXX, to display a preview image in the window handle whose numeric value is described by the XXX.