.png backgrounds solid black in iMovie

I've read the various suggestions but nothing has helped. Installed latest iMovie ( 10.4.3 ) running with Sonoma 14.6.1 but still can't import .png, or other files which ought to have transparent backgrounds, from Photoshop. That which should be clear is completely solid black. Sooo frustrating I've been using the technique for years to create animated paintings but not now when I've acquired a Mac with sufficient punch...

Is it just me?