Hi Taja,
Picture size is not set in stone, but you can't go wrong by sticking to the rule of
'smaller is better' and reduce your pictures as much as possible without compromising picture clarity and quality. I reduce all of my pictures by 60% from cropped (if required) original, using Photoshop. The total size of your page (kb's) is the directly propotionate to 'load speed' and the biggest culprits are picture size and background music etc.
Have a play with this wonderful
picture resizer from VH.
Cheers,