Often you will want to capture a shot of something on your computer screen to use as an example or to show to someone etc.
On a mac this is easy:
* To capture the entire screen, press Apple/Command+Shift+3.
* To capture part of a screen, press Apple/Command+Shift+4 (the cursor will then turn into a cross-hairs icon. Click and drag over an area to select it. When you release the mouse button, the image is taken, and you'll hear a snapshot sound)
You will then find the screen shot is saved to your desktop as a photo. You can then crop/add photo effects etc and easily upload the photo to your blog. Don't forget however to change the file to be a jpg by going to File>Save As and choosing >jpg.
See this article for more info on taking screenshots on your computer.
To see an example of how I effectively use screenshots on my blog see this post.