Is there an app that allows users to take multiple photos and upload them all together? I see most of popular iOS apps allow only upload one picture at a time to the service's backend storage. Is there any reason why they don't support upload multiple pictures in one shot?
Why yes - the iOS photo picker has been modal for some time now and allows one picture to be selected by apps that use the long time iOS API to access the camera or the picture roll and albums.

Furthermore, copy and paste is also a single item and there is no clipboard history functionality in iOS.

I suppose a jailbroken device could be modified - but I don't know of any apps or work to actually do that, so this is a theoretical capability to my awareness.

You can use the Photo all (and the new iPhoto app as well) to send multiple photos to email or a message. In that case, you can email these multiple pictures directly to a Posterous Spaces account that you have set up using the Posterous app. Twitter has purchased Posterous recently and announced that the service will not be shutting down immediately and that users will have an export tool and notice should the service get shuttered in the future.