Have an entity P and an associated image-entity P_Image. Have a form containing a data grid for P and a New and Edit button. On the NewEdit form I included the name/image/upload frame over the association. No errors. In the browser I select the P, press Edit and the NewEdit form is shown. Also I get a red cross for the image (no wonder, it is not there yet) but i can't click the 'upload image'. ANy idea what I did wrong?
It sounds like you haven't yet created an instance of object PImage. When you click the New button, Mendix generates a new, uncommitted instance of P that displays in the form. But Mendix does not create an instance of PImage.
This will likely take some microflow work to ensure that you have an instance of P_Image created for the new instance of P you are trying to create....