Questions about photo uploading

Hi, im a bit of a newbie at uploading and resizing my screenshots and I was just wondering a few things.
First, the photo has to be 100k, and some of mine are over 5mb. How do I reduce this?
Second, is making the screenshot 800 pixels wide as simple as cropping in photo shop or paint?
Thanks,
Nigel
First, the photo has to be 100k, and some of mine are over 5mb. How do I reduce this?
Second, is making the screenshot 800 pixels wide as simple as cropping in photo shop or paint?
Thanks,
Nigel