I use the terminal for many things, but it's less than ideal for browsing images. Still, sometimes you need to check an image in a folder, maybe before adding it to git or just because you love that cat.gif.
The tool qlmanage (which is included in OS X) allows you to do that. To make it a little bit easier to use, just add the following code to your .bashrc or .zshrc (whichever you use):
alias ql="qlmanage -p"
Reload of your config (open a new terminal or do a quick
. ~/.bashrc) and you're all set. From now on you can call
ql cat.gif or even
ql *.jpg to open a Quicklook window. You can even navigate between images with your cursor keys!