Just a few questions to help clarify the problem.
Do you get this in your development environment as well? And what do your log files say? Can you find a stacktrace in them related to this exception that you could share with us? Also, have you made any changes to the environment that you can think of? Installed EPiServer patches, updated dbs, and so on..
Is this error appearing on all pages where you can add images or is it an isolated occurence?
Sorry for late feedback! It seems it was a custom ui property that caused the error. The property was named almost the same as the standard property which confused me. The error message was hard coded to be thrown no matter what happened. Thanks for your assistance though!
No problem :) Good that you figured it out!