Gozaimasu or gozaimashita

Is the latter used for past tense? When I was in Japan, after I took a photos of people, I said "arigato gozaimashita" but they always looked at me funny, and when I said "arigatou gozaimasu" to others, in contrast they didn't give any strange reaction.

I was wondering if I was doing something wrong?

Yes, the "shita" is the past tense.

I don't think you said anything wrong... Personally I don't find anything wrong with saying "Arigato gozaimashita" after having taken photos, because what you are trying to say is "Thank you (for HAVING ALLOWED me to take photos)," so thanking them for something they've done for you, so to say.

But because it is in "past" form, (1) some people might have found your thank-you sounding a bit too final, as if to end the conversation. Or, (2) some might have wondered "Have I done something for you before?" because it is often used to thank others for past favors given, like "Senjitsu wa suteki na purezento wo arigato gozaimashita" (Thank you for giving us a nice present the other day).

As AK said, "gozaimashita" is right.
If I asked for permission to take photos of people,
also I'll certainly say "Arigatou gozaimashita" after I took photos.
"Arigatou gozaimasu" can be allowed but a bit strange because it's done.

Although I can't find the reason of thier reaction.