The phone number was the only ID registered for iMessage. I clicked on the e-mail address and got an error that it wouldnt be registered for iMessage. After that I couldnt change it back to the phone number because the error popped up everytime.
So I restarted the programme and the phone number was automatically chosen again and I could send the picture without problems.
If anyone else experiences this problem, try 1) selecting a different ID of the receipient, eg the e-mail address (even if its not registered for iMessage) then 2) switch back to the phone number, or restart imessage to switch back to phone number.
This at least worked for me.
If the 0742xxxx is a partial phone Number it may pay to list it in your Contacts Address Card as an International number.
If it works on an iPhone as the Phone number for them it is obviously local to your country and you probably need to add the Country Code.
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.3)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously