If one photo is being referenced, credit should be in all caps with a period at the end:
If multiple photos are being referenced, credit should be in all caps, identifiers should be in ital, and there should be periods in between listings and at the end of the last listing:
PHOTOGRAPHS: THIS PHOTO: ISTOCK.COM. OPPOSITE LEFT: GETTY IMAGES. OPPOSITE RIGHT: NORTHCUT.
It’s OK to use identifier other than directional if it makes sense (example: product stories):
PHOTOGRAPHS: BACON: GETTY IMAGES. YOGA MAT: JAY WILDE.