Screen Shots

If you want to capture an image of anything on your screen, take a screen shot:

on a Mac
(to copy the image into a new image file for uploading directly to the web or to Moodle, or to insert it into a web page):

With the image you want visible on your screen, hit Command+Shift+4
Use the cursor to drag diagonally across the screen to select the portion you want in your screen shot
When you release the mouse button after making your selection, the image will be saved as a .png file (or other web-compatible format) on your Mac's Desktop.

on a Mac
(to copy the image into the Mac's clipboard to paste into a software document--e.g., Photoshop, Word, etc--for editing or layout):

With the image you want visible on your screen, hit Command+Control+Shift+4
Use the cursor to drag diagonally across the screen to select the portion you want in your screen shot
When you release the mouse button after making your selection, the image will be saved as a .png file (or other web-compatible format) on your Mac's Desktop.
See the MacRumors page about the various options and directions for taking screen shots.

on a PC

Hit the "Prt Scr" button on your keyboard
In Photoshop or some other image-editing software, open a new file
Paste into the new document (hit control-v)
As desired, crop and size the image
Save the document as a .jpg or .gif to use online
(In Photoshop, for example, choose File > Save for Web)