You can't just paste them into the form, but you need to paste the URL to the image.  The easiest way is to press the image button on the form, and then paste the URL to the image into your message.  For example, if I use

[code][ATTACH=full]62810[/ATTACH]

[/code]

it will include your image directly into the post

[ATTACH=full]62811[/ATTACH]