On the computer the best option would be to create a free photobucket account, upload the images to that and it will then give you the img code which you copy and paste into the forums and your picture will display. The code you need will look like [img]filename and text [/img]
I sometimes post as an attachment to the message. That way I don't have to upload the photo to a 3rd party site. I THINK (not sure) that that way the image remains on the site even if the 3rd party site goes down or the image is lost???? I've seen threads where the image is no longer available making it hard to follow - I think that's because the site or link no longer finds the photo.