Images should be sized in according to the following guidelines:
Portrait image – 213px (width) x 320px (height)
Homepage carousel image (template option 4) – 600px (height)
Homepage carousel image (template option 6) – 375px (height)
Interior page banner image – 1200px (width) x 600px (height) is suggested to fill the space
Resources for how to resize an image:
Using Photoshop – video tutorial
Using Photoshop – step-by-step guide with visuals
Using ResizeImage.net – a free online image resizer and compresser