"There are 1,500 cat photos in my cellphone."
As a native English speaker I would only say "on my phone" just as I would say "on the TV" or "on the computer". For something like a battery or SIM card I would say "in" though.
Again with that extra "的." It would make sense if it was 张照片的猫" because that would be "photos of cats," but the extra "的 in the accepted sentence I only put because it was provided. I don't really understand why you would say "猫的照片"
As I understand it, in this sentence the 的 is not being used to indicate possession but is used to indicate that 猫 is an adjective before the noun 照片.
I don't understand why it is cats (plural) and not photos (plural). Can anyone explain this?