Brian

You can have up to 3 different sized images - The thumbnail, the "normal" image, and the original (if you've selected to save original images). If you need to resize your current images, first make sure you have the dimensions set as you would like through the 'Edit Filetypes' link in the Admin CP, then go to the 'Maintenance' link select the option to rebuild your image sizes (be sure to select 'Normal Images' from the select box), and that should resize them for you.