You are linking to your own computer drive. It's probably easier to upload the photo to the photo gallery (see link to Photo Gallery near top of this page).

Then when you post there will be a "Photos:" option from which you can select the photo to be included with your post.

For example, I will select a photo of my male blue spiny lizard by highlighting it from the Photos: list. I will then use "Preview Message" to make sure it's there, and then "Post Message" to put the message on the forum.