You don't use the attach feature to post a bigger picture.
Load your (big) picture to your ssnakess gallery (go into Gallery, then Member, then upload or add a pic or something like that).
Then go into that picture's page, and right-click on the picture. Select properties, and copy the image URL.
Now go to where you want to post and type (IMG)paste url here(/IMG).
Replace ()s with <>s, and you're all set!