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?
It all depends on how the app developers chose to implement the software.
Take Dropbox as an example. It's pretty popular and let's you upload multiple photos at once:
In fact, I would also expect them to eventually make it automatically upload all your photos as part of their auto photo upload feature they are rolling out.
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.