Most addin pages should have at least one screenshot. This is used to give people a preview of the addin, a little taste of what the addin looks like. It also makes the page a little more colourful and appealing.
This page explains how to take and save screenshots.
In Windows, you can take a picture of the screen using the Print Screen button on your keyboard, which is usually written as PRTSC or PRTSCN. On some laptops you may have to hold down a Function (Fn) button while pressing the Print Screen button.
When you press Print Screen, everything you currently see on the screen is copied to the clipboard, which you can then paste into a program such as Paint. You can then save this as an image file. For goomod screenshots you should save this as either a JPG, PNG, or GIF file.
So to take a screenshot of your level, play your level, and when you want to take a screenshot press Print Screen on your keyboard. Then exit World of Goo, paste the image into Paint, and save as one of the file types mentioned above. Then upload it to your addin page and you're done!
(On some graphics cards, World of Goo may need to be in windowed mode for you to take screenshots. In this case, you can Alt+Enter to change to windowed mode and use ALT+PRTSCN to take a screenshot of only the active window.)
On a Mac, you can save a screenshot directly as an image. Press Command, Shift, and 3 at the same time, and it will automatically save a picture of your entire screen to the desktop as a PNG file.
So to take a screenshot of your level, play your level, and when you want to take a screenshot press Command+Shift+3 on your keyboard. Then exit World of Goo, upload the image on your desktop to your addin page and you're done!
(On some graphics cards, World of Goo may need to be in windowed mode for you to take screenshots. In this case, you can press Command+Shift+4, then space, then click on the World of Goo window.)
It depends on the distro, but generally identical to the Windows instructions (PrtSc or Fn+PrtSc to copy the screen to the clipboard). Some distros (KDE-based ones, for example), will pop open a window you can directly save the screenshot from. If not, just paste into GIMP or your image editor of choice and save as PNG, JPEG, or GIF.