Problem with the display of picture

Hi to all! (and sorry for my english..!)
I have a problem that don't know how to solve.

I have a form consisting of:

1 groupBox that contains, inside, 4 pictureBox (which we call to understand pbPrimary).

Both the GroupBox that the PictureBox have the background transparent, so that you can see the background of the form.

"above" each pictureBox, I put another pictureBox (pbSecondary) which also has a transparent background, but are smaller than pbPrimarie.

The pbSecondary simply contain images .png of a cockade. The image has the "transparency" on the color white, so that to give the appearance that it is "glued" on the background image (in this case, the background image that i want is the image that i show in pbPrimary)

During the execution, in pbPrimary appear the image of a guy and later a cockade on the picture of that guy.
What happens is that the cockade that is in pbSecondary, which has a transparent background, DO NOT take the background of pbPrimary BUT 'to the form!

I wish the background of pbSecondary is the image of pbPrimary and NOT the background of the principal form.

Certainly, i don't have imposed something, such as the "owner", or some "priority", but i don't know what needs to be set.

I hope i was clear! Thank you in advance for your answers!

Massimiliano C.