Some of the links on this forum allow SMF, at no cost to you, to earn a small commission when you click through and make a purchase. Let me know if you have any questions about this.
I'm sure there are plenty of other thread discussing pic sizing and in the old forum I always shrunk mine before I uploaded them to my host. With the new site it's easy to just upload them to the forum and then while you're in your post edit, right click on the picture and hit properties. You can adjust the size right there. I usually set the horizontal at 600 or so.