To create an eye catching desktop background, you only need a beautiful image, such as a photograph or piece of art, and an editing program designed to change the image's resolution to match the desktop's resolution. As long as the resolution matches your screen resolution, any image you choose works.
The first step to is to get your resolution. This can be found from the Display Properties menu in Windows, or from Display in System Preferences on a Mac. Look for two numbers with an "x" in the middle, such as 1280x800 or 1920x1080.
Next, choose an image. It can be a photograph you've taken, an image you've found online, a piece of art you've made or anything, as long as it's clear and free from blurriness and artifacts (small areas with color problems). Any kind of distortion on the image becomes more exaggerated when the size of the picture is changed and causes it to look messy.
Finally, open the image in an image editor. If you don't have one, GIMP is a free, fully-featured editor that allows you to change the resolution of an image. Once you've opened the image in an editor, choose the Scale Image, Resize or Image Size option, and then enter your screen resolution as the size and uncheck Constrain Proportions. If the numbers change in tandem rather than one at a time, you have to uncheck the chain icon next to the resolution. Once you're done, click OK, and save the image.