The fact you were able to upload those other images suggests there's nothing wrong with the attachments feature, otherwise you wouldn't have been able to upload those pics of the forum itself.
But the image of the babe is 10MB and the file size limit here is 2MB. Anything above that will be rejected. You will probably find that your image was too large as well
@RealDarko
Anyway, it is a shame that the error message doesn't get straight to the point and tell you that it is too large, but I nevertheless think that is the reason. I will see if we can control the error message wording, but I'm not sure that we can.