That's a limiting factor of the forum's browser-based environment (in this case, vBulletin)... and not the iPad. Actually, I'm not even sure Google Docs is able to allow pasting of image content into the browser text-box... at least, it didn't work in Windows just now (but this is with IE... I dunno if it has that feature in Chrome or Safari or Firefox).
Also please remember to post threads in the appropriate locations. Threads about how to use this forum belong in Site & Forum Feedback. Thanks.