If you want the NC wallpaper to scroll across three screens, then make it 1800px wide by 1024px high. Basically you are creating three 600w x 1024h wallpapers in one image.

I find that sites like interfacelift.com have a lot of HD sized wallpaper (1920px x 1080x) which can easily be altered by many imaging software apps to 1800px x 1024px. You only need to slightly reduce and trim HD wallpaper to make it a perfect fit and scrollable on the NC.

Whether you want the wallpapers to scroll or not, just remember that the NC is considered a portrait view device (600 x 1024) whereas a netbook with similar screen size is considered a landscape view device (1024 x 600), so the netbook wallpapers will be sized for a screen that is assumed to be rotated 90 degrees from the NC. Yes you can view the NC in landscape, but a single wallpaper for it will need to be 600 x 1024 rather than vice versa. If you don't mind having a landscaped wallpaper, then you can take one designed for a netbook (1024 x 600) and use image software to rotate the image 90 degrees counter-clockwise to 600 x 1024. Windows picture previewer can even do this and it is built in, I'm not familiar with MACs but I suspect they come with software that can also do this.

Here are three examples of image orientation for single-screen wallpapers, and one example of a scrollable (1800 x 1024) nookcolor wallpaper:
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