I've seen many good pictures in ways I did not think I'd see on this wiki. For a great example, look at the picture of the TNT from "The Time Lab" by "bobisdacool1" (it's a great level btw). It has no background, which is what I never expected., but how did they pull it off?
So now you know what to look for. I have two ways to do this, however, I will list the one for other users' levels first, as I bet you will see it more.
For others' levels
If you found this cool level you like, and there's an item you want to take an image of, first of all, that's great! I rarely see levels like that nowadays. I'll use the first rock on the "Happy Green Hills" stage for an example. If it's on this wiki, then slap the tab button and click the fullscreen button in the bottom right of the menu. As the pop-up likely said, press the escape key to close fullscreen when you need that. Now, get up to the object and press Ctrl + Alt + Printscreen and open up your favorite art package (I will use paint.net). Now, do a rectangle crop on the object.
I've gotten that done, but now I'll use a brush/pencil tool to make the outlines white. Now, fill in the outside with the white fill tool. I ended up having to use MS Paint, which meant I had to use a filled square tool, but it did the job. Now shrink it, but I won't for the purposes of this tutorial.
Now I can upload this at (almost) the highest quality!
For your levels
Taking pictures of your levels is, thankfully, much easier. I'll use the Danger sign from my Obstacle Course.
First, load the level in the level editor, and now, to go fullscreen, you put your mouse on the thing (I can't classify it as menu, or button) and then check "fullscreen". Again, use the escape key to close fullscreen. Now, take the object, and clone it. Yep, you heard me right, clone it with Ctrl + C and then Ctrl + V. Now, remove signs of the editor such as joints, and make sure your object is against the white editor background. Zoom in as far as you can go while still having the full object visible, and press Ctrl + Alt + Printscreen.
Now with the image stored, open up your favorite art package and crop the image onto the object, and shrink it (unless your object is like, microscopic). As I said, I won't, so you can glare at the beauty of the high quality image. If there's text, use your art tool to fix the lines.