The user should be allowed to select an image from more than just their device.
Countless services provide this functionality, Slack is a great example. Is there a library that can assist in this as my searching has turned up blank.
I really don't mind if it's in Objective C or Swift.
In answer to your question, the library used to generate the action sheet included in the image you attached is a native iOS library / class called UIDocumentMenuViewController.