Attachments
Attach a logo or a set of photographs to a message, drag them onto the chat, or paste them straight in. They arrive as part of the request rather than as something to upload somewhere else first.
A brand file that sits on a solid background is offered a transparent version, shown as a before and after, and it takes a nod from you before it is used. The original is kept either way. Which makes a rebrand about as long as it sounds: attach a logo, type rebrand, done.