Blend tool doesn't allow me to 'make' blend

[Daphne5EE3]

Hi! So I'm quite new to Illustrator. For some reason, Illustrator won't allow me to make a blend between two pictures. In the menu, 'make' is grayed out. Creating a blend between two shapes, however, is no problem. Does anyone know a solution?

[Monika Gause]

Those are pixel images. It only works with vector artwork.