Have you ever found that super cool wallpaper that you had to have, applied it, then find out that it looks like crap on your desktop, notebook, or mobile device? While stretching the photos of your kids and using them as backgrounds is suitable for some, I prefer high quality backgrounds that don't look like they've been stretched out.
First you need to know what the resolution of your monitor is. Your monitor consists of tiny little individual dots or light elements. You can think of them as very small LED lights inside your display case. These are called pixels. If you could take a magnifying glass and count the number of pixels from right to left that would give you the first half of your resolution. Then you would count the number of pixels from top to bottom giving you the second half. For example my 25" monitor is 1920 x 1080. That would indicate that I have 1,920 pixels from right to left and 1,080 pixels from top to bottom. That is my resolution.
I don't really expect anyone to count the pixels in their monitor unless your that OCD and just have to know. So there's a couple of quick and easy ways to find out. One is to simply look at the paperwork that came with your display or check the manufactures website. Second you can check in the display options on your computer. With the number of people using different operating systems, I will leave it up to you to find out where your display properties are. If you can't find it, simply click the link below that says "What Is My Screen Resolution" and it will tell you.
"What Is My Screen Resolution"
Now that you have your screen resolution you can go and find the right size wallpaper for your desktop. Only problem here is that chances are you won't find the perfect desktop wallpaper in your resolution. Luckily you have a choice here. As long as you stay within the correct aspect ratio, the wallpaper of your choosing should still look good. For simplicity's sake, I'm not going to go into aspect ratios but if you follow the image provided and stay within your color family, you shouldn't have a problem.
For example if I have 1920 x 1080 display and I look at the chart, the color of that box is blue. So that means that any wallpaper I choose should match the resolution of one of the blue boxes. One that would work 1280 x 720. I don't recommend that you go more than 2 boxes up or down. The difference in resolutions will cause a grainy appearance more than likely at this point. Meaning that I shouldn't choose 854 x 480.
Keep in mind it is still best that you get the wallpaper that matches the resolution of your monitor for a number of reasons but this guide will help you out if you cannot find one that matches.
More detailed information on display resolutions can be found on the Wikipedia page.