easy ..just look benith the reply text box, click "go advanced"..
then under the lower "additional options" there is "manage attachments"...click that & browse to the file picture you want to upload directly to the site....there is also a "key" which helps you with the max file size etc...
I usually go for jpeg 1000x1000pixels max dims..& just under the 1MB file size...nice & considerate size