Elendil, olddad has pointed you in the right direction for uploading images.
The file however should be 200kb or less so that those on dialup can view it more easily.
In the addreply box, scroll down towards the bottom and choose the Browse button to find the file on your computer, then click Add Attachment to add it to you post. From there you can click the ADD TO POST button which will give a thumbnail image into your post.
Hit ADD REPLY and you're done!
Let me know if you have any problems and I'll try to be more specific.