It takes a little work but:
Generally, never post an image larger than 1000px for either Horizontal or Vertical.
You can resize large photos or scans on Microsoft Paint.
- Open the image with Microsoft Paint
- "Resize" at the top left
- Select "Pixels"
- Usually 700 - 750 for the longer side is best
Upload using http://www.tinypic.com
(You can resize here as well: Message Board (640x480))
Upload your image, then copy and paste the:
"IMG Code for Forums & Message Boards"
You can also do a Google Image search for images:
Google Images, click on the image, click "View Image", copy the link at the top of your browser, paste it using above.
Other sites to upload:
http://postimage.org/
http://www.photobucket.com (if you use Photobucket, be advised the image will disappear after a period of time)
http://imgur.com/