I see: If you want to use the image as wallpaper in Windows, the only acceptable file types are BMP, GIF, JPG, DIB or PNG. (You can also use HTML, but that's not relevant in this case).
Of these filetypes:
- GIF is out, because it only supports a maximum of 256 colours - thus rendering gradients very poorly.
- BMP is an uncompressed format (the standard Windows raster format). However, Windows uses a fixed colour palette for BMP files which cannot be changed so transferring an image to BMP format may result in some unwanted colour shifts.
- DIB stands for Device Independent Bitmap. This is basically the same format as BMP but without the 14 byte BMP header.
- PNG format is similar to GIF but supports lossless compression and 24-bit colour. There is a well-documented issue with Internet Explorer and transparency in PNGs, but if you're not using transparency that's irrelevant.
- JPG is probably your best choice as long as you follow the correct procedure:
- Do all your editing before you convert to JPG. Editing and re-saving a JPG will introduce artefacts, and the more times you edit, the more artefacts you'll get.
- Select File > Save As... rather than File > Save for Web and Devices... (unless you want to try out different levels of compression or see how your image would look as a PNG). Choose the highest Quality setting (12).