To take a screen shot, you can do one of two things.
If you use razor, under the screen shot tab, click the Take Screen Shot Now tab. This will add a screen shot to your Razor/Screenshot folder. You can choose the entire pc window or your UO window only.
You can also take a screen shot by clicking the print screen key on your keyboard. This will copy what you see on your monitor to your computer clipboard. To save the screen shot you will need to open MSPaint or some other graphic progam. Use control + c to copy the picture to the program. Save the screen shot to your computer.
Now that you have your picture, you need to get it on the 'net. Sites like tinypic.com and imageshack offer free hosting. Follow their instructions to load your screen shot/picture and get the link.
Now you can add your picture to your post.
Choose the insert image tab from the advance menu and paste the link in the field. Or paste the link directly in the post and wrap the link with image tags [IMG] before and [/IMG] after the link so that it will be
[IMG]link here to picture/screen shot[/IMG]
For a visual guide, visit Pepsi's guide on the Hybrid forums.