You're tying to embed the image within your post. In order to do that it must be available from a public Web server.
To attach one as a link, see below the composition window when you're making your post. With that mechanism you can upload an image with your post. You should wait until you're ready to click the "Submit Reply" button, though, since if you add the image and "Preview Reply" the image will be sent, but in the window that returns it will be absent from the dialog box. If you click "Submit Reply" at that time the image will not be part of your post unless you again enter the location first.
Another option to having the image embedded with your post is to submit it as an attachment and either reference that attachment in a subsequent post, or go back and edit the first one with the now-known location on the Web for the image.
This will be an example of that last method, though it should be used with discretion, since some of us modem users can be inconvenienced by large or multiple images.
In any event, you'll do very well to keep the image size to something less than 100KB. The quality of the image does not need to be anything more than about 75dpi and that will affect the size in bytes of the image more than it's "physical" dimensions.
Glen
<img src="http://www.arboristsite.com/attachment.php?postid=150337">