I did something similar in the last couple of days and it works for me. I have a 1-1 relationship defined between main entity and picture entity. Try that and see if it works.
See answer of Michel Weststrate in question 1126.
In short; I did not store the association to the image. In 2.4 this construction works, since nested dataviews (in the edit form) automatically create objects and set associations. But in 2.5 this is no longer the case (see the migration notes for more details) and you need to set the association 'by hand'.