CSS background image on background color

Question

If this panel is chosen, a panel that I coloured blue will appear (clicked on it). In addition, I put a tiny symbol (.png picture) to that panel to show that the currently selected item has already been chosen.

The user will be aware that he has already clicked on certain panels if, for instance, he sees 10 panels and notices that 4 of them have this small indicator. So far, this is working well. The issue currently is that I am unable to both display the small sign and turn the panel blue simultaneously.

I changed the panel's colour to blue using the CSS property background: #6DB3F2 and the background image: url('images/checked.png'). However, it appears that the backdrop colour is above the image, making it impossible to see the sign.

Is it therefore possible to set z-indexes for the background color and the background image?