As mentioned,your photo-program or photo album where your pics are held should have a feature to "Edit" and in the edit menu there should be an option for resizing to whatever dimensions you choose. ( approx. 500x350 pixels seems a good average size to me, but everyone has their own preferences.
If you dont have this feature at all,or are unable to get it to work, you could consider asking another with a photo program to do this for you.
Email them the pics for resizing, and once sent back to you, they are ready to go up.
Looking forward to seeing your pics!