I want to share a foto with someone via MMS. When I've picked my picture and pressed Send, It takes a minute or so and than, on the place where the picture needs to be: Can't send message. Again. I pressed a few times on Again but it kept saying that message. What now?
SMS (Short Message Service) doesn't support pictures or other similar content, only plain text. Perhaps you're referring to MMS (Multimedia Messaging Service) instead?
In order to send MMS messages, you need to do two things:
Configure the MMS APN (Access Point Name) settings. You can do this manually in:
Windows Phone 8: Settings → Mobile network + SIM
Windows 10 Mobile: Settings → Network & wireless → Mobile & SIM
Many carriers also support sending the necessary settings to you automatically. Look around on your carrier's website for instructions on how to request the settings.
Enable mobile data. Without this, MMS messages won't be delivered.