Changing picture size

From iPhoto 5.0, select the picture, click on the Share menu, go to Export and from there it's pretty self-explanatory.

 

You can do it in iPhoto.
Just import the image, then export and change the settings to what you want.

Edit: mad_jew! You beat me.

 

The iPhoto suggestion above is best if the photo is in your iPhoto library.

Theres a cool free app called Toyviewer that I use for quick jobs outside of iPhoto.

 

Alternatively, from Preview, you can go to the File menu, select Save As and both change the file format and its quality however this method is pretty vague. It just provides you with a sliding scale and as far as I can tell, no indication of any parameters it might be using.

 

I'm just lucky.