More Mac 101, our series of tips for new and novice Mac OS X users. Here's a tip that I was recently reminded of while choosing photos from a web browser for upload. Mac OS X Leopard
Let's say you're looking for a photo to upload to Flickr or similar service. With your browser open, select Open from the File menu to invoke the Open Dialog Box. On the left hand side, select Media. Now you can browse your iPhoto library and even scrub through Events if you have iLife '09 installed. From there, select the photo you want, click Open and you've got it.
There are also a couple of tricks available that aren't immediately obvious. First, control-click (or right-click on your mouse) any neutral space in the dialog box to bring up a contextual menu with the option to switch between icon view and list view. Once in list view, the contextual menu adds options to open a file in its parent app, reveal its location in the Finder and display information columns (name, size, kind, etc.).
Additionally, hitting the space bar with an item selected highlights it, similar to Quick Look. A photo is brought to the forefront and music and movie previews are played.
It's handy, often overlooked and a great way to ensure that you've got exactly the file you want.