setting up a background photo

Holding down shift will allow you to select multiple things- or holding down 'apple' will allow you different control- you'll see what I mean. If in list view, holding down shift will select the first thing you click, the last thing you click, and by default everything in between- with 'apple' it will not select items in between, just the ones you click on.

 

Best I know, you have to set the picture using system prefs -- desktop. But you can set it to change at a preset interval, which can be fun.

Command-click (apple-click) allows you to select multiple files, and works in a number of different contexts.

 

If you right-click (or control-click) on the background pic, there's an option to change it. You'll be brought to the Desktop & Screensaver preference pane. You can pick one of the supplied pics, or make a folder of all the ones you want and choose that (it's not recursive though ).

To select multiple items, command (the key with the Apple and clover thingie) click on what you want to select. Or, if you use list or column view, shift clicking will select everything from the top- or bottom-most selected item to the one you click on.

 

Here's some tips for PC->Mac switchers:
Generally, anything you did with the Control key on the PC is accomplished with the Command key (also called the Apple key - it has a cloverleaf and the Apple logo on it) on the Mac. For example, selecting non-adjacent items and using keyboard shortcuts are both done with Control on PCs and Command/Apple on Macs.
Also, unless you bought your own two-button mouse, anything you accomplished with the right mouse button on the PC is done with the Control key on the Mac. If you have a two-button mouse for your Mac, then both options work (the PC way and the Mac way).
The Shift key behaves the same way on both platforms.
The Option key is Mac-specific - its purpose is to ease typing international characters. For example, to type 'é' on a PC, hold down the Alt key and type 0233 on the numeric keypad. On a Mac, hold down Option and press 'e', then release Option and press 'e' again ("option-e e" for short).