Displaying MDI child form

I have this parent MDI form, which I have covered up with a picturebox. I created 2 buttons on this picturebox, OK and EXIT button. When I pressed the OK button, I want to go to a MDI child form. But it seems that the picturebox is blocking the child form. What should I do?

Hide the picture box,

or open the form but not as a child but as a modal window.
It will then go on top of the picture box but unfortunalty also the whole form.

Grimfort