When you upload images from your digital camera to WordPress you end up with huge files. Thankfully there is plugin to fix this which works just great: Imsanity

Imsanity can limit the resolution of all images you upload. Just specify a maximum width or maximum height. It can even resize existing pictures that were uploaded prior to installing Imsanity and it has support for WordPress MU (multisite / network installation). You can configure the settings via Settings > Imsanity. Here a screenshot:

resize wordpress pictures