If an actor does not have a photo downloaded via script, you cannot enter one manually. The filename comes up as .jpg, _1.jpg, _2.jpg etc extention only and does not import the pic. If the actor does have a photo via script, adding others is no problem.
This is copied from import window:
Selected File : C:\Users\James Rosendahl\Pictures\1_Working\Monica Leigh\monica-leigh-05.jpg Loading Image to Control : C:\ProgramData\Binaryworks\XMM8\tempdir\images\monica-leigh-0520121007145446199.jpg Saving Image to Disk... Chosen Title: Covers Directory: C:\ProgramData\Binaryworks\XMM8\databases\My_Movies_photos\ ERROR DETECED ON COPY FILE: ErrLevel:1-91-Object reference not set to an instance of an object.
Have you tried adding images with "Add a Cover at First"? To me it seems that if you use the Cover Manager it will not work but if you add the first picture via "Add a Cover at First" XMM8 creates its desired filename and afterwards you can even use the cover manager to add more pictures. It's just a workaround but maybe it helps.