Allow add thumbnail when adding image

[astralbodies]

Original issue: https://github.com/wordpress-mobile/WordPress-iOS/issues/1255

When adding image it displays at the size selected. Should import full size image and then have option to display thumbnail or medium as with .org. Reason is so that on the blog people can click thumbnail and view full size image.

[doomi]

there should be a setting to define the default "web display setting" of images when creating posts (Thumbnail, Medium, Large, Full Size). currently, every single image in a new post has to be edited to change it's web display size.