Quote - Xandi:

1. Select Insert/Edit Image at the top of the message post box.

2. Enter the URL of the image. Then click on Appearance tab and enter 600 in the first Dimensions box. Make sure Constrain Porportians is unchecked and the 2nd Dimensions box is blank.

3. Once the image has appeared in the post message box, click it and then click on the Insert/Edit link icon. Enter the image URL again, and select Blank New Window.

That's it :)

That's a lot of work, just go to the Attach a File below the message  and browse to the image location on your computer. It's the same as Daz, keep it under 500 kb..

Edit:  If you want to post the full 1or2 mg file you will need to go the other route.