It depends on how Apple implements the iMessage photo picker. If they go
through the document picker API, you will get a chance to provide images.
If the use the photo picker view controllers, however (PhotoUI framework or
otherwise), you will not be able to.
Apple is most likely to go through the second route, but if this is an
important feature for you, make sure to file a an enhancement request with
Apple and post the radar number here so people can duplicate.
As of beta 2, Apple does not use the document picker API, so you will
not be able to add your own data source for images if the Messages app does