You mean uploading images to place in forum posts?
I am not sure how to directly upload images from your computer onto a post, but what you could do is use Photobucket as an image host, upload your images onto there, and copy/paste the URL for the image into the field provided after pressing the Insert Image button. The Insert Image button is present in both the quick reply and normal reply, and it looks like a square picture of gray mountains with a yellow sky background.
If you are uploadiing them straight from your computer you need to click the button under your editor (the thing you type in) that says "go advanced" Then scroll down and you will see a button that says "manage attachments" click on that. You will get a pop up box and at the top is where you can upload the image. Just click "browse" and then tell it where on your computer the picture is that you want to upload. The pictures can only be 800X600 and I think there is a size limit too.
If you are using a host such as flickr or photo bucket then all you need to do is click on the button that looks like a picture (it's yellow and has what looks like mountains on it). you will get a pop up for you to link the web address of your photo too.