It's all just a case of native speakers not sticking to the correct tenses. It probably stems from our frame of mind: the pictures were taken in the past vs. the photos still existing in the present.
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Anonymous"Photographs without credit are taken by me."Both are and were are completely acceptable. You are taking words from different parts of a longer statement that is implied:
"Photographs without credit were taken by me."
"In the future, if you see any photographs without credit, they are taken by me."
"In the future, if you see any photographs without credit, they were taken by me."