Well after further testing and resetting write permissions this looks like a bug.
I'll try to give as detailed info as I can:
Types Version 1.2
I have attached a single image field to the regular post
I can initially upload a picture, that will save properly.
When changing the image source to emtpy string and updating the post, the field is empty but the post still has the initial image attached.
When changing the image via WP interface (replacing the string in the image field), and updating the post nothing happens.
When uploading a new image to change the initial one, a second image is attached to the post.
When uploading even another one it also gets attached to the post and so on.
I am sure i have set the custom field to a single image value " This field can have only one value".
The field is also set to "not required"
As said, this makes the whole (apart from this) perfect plugin unusable as it is now.
Cheers,
Junta