The first bit of text on the main window under picture of the week says.

SEND US YOUR PICTURES! click here for the upload form. Especially needed right now are poison oak and poison sumac pictures.

The upload form allows you to send us your pictures. Then use the post a new submission link. The content is your textual description and use either the image or attachement button. Whatever is easier. If you have mutliple images you can zip them up and use the attachement area.