I need help.

Awhile ago I decided to change the picture in my header. Now it's been a year since the previous one went up. I did what seemed like the logical method to get a picture up............... to big. Tried again, even bigger....no good. Tried again, baby cried, 2 hours of sleep the night before finally got to me..........left it that way. I've been assuming it happened because I was using iphoto for the first time and I'm still a bit challenged there. Well, here I am back in the old pc, reverted back to the same old picture AND .............................I have no idea how to make it smaller and fit across the header without being huge. Any advice?