You could use a unlimited string, base64 encode the image en encode when receiving?
@ Liouwe, we'd like to check this out. We can help you if you send your project to us (stripped from private information of course). Please send to firstname.lastname@example.org
To elaborate on what @Stephan is saying; If Mendix is the receiving side of your request then the side that is sending the image whether it is a mendix or none mendix application must send the image encoded in base64. I have ever done this with android and you can use base64 to send any kind of binary media to mendix webservice.