If you are being asked to describe a picture, what tense would you use?
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I would use the present tense to describe the picture itself. Reserve the past tense for talking about the subject if it's an old picture.
See? Present tense for the picture, past tense for the subject.
Use present tense to talk about the subject matter if the picture has just been taken. It's not cut and dried when you should begin talking about the subject in the past tense, but if I just take a picture of you on my cell phone, I might say, "You're not smiling enough. I'm going to take another." If it were, say, an hour old, I might say, "I wish you'd been wearing your blue jacket. It would have gone great against the yellow wall. Still, I'm not going to delete the picture. It's a good record of our friendship." And so on.
If by pictures you mean "paintings" the same rules would apply, only you are less likely to talk about the subject matter in present tense.
When this picture was taken, it was a grey day and had rained for 3 hours. I was completely soaked as you can see
In this picture you see me completely soaked after 3 hours in the rain
I used past tense for reviews, because you already read the book/seen the movie and the general idea is that you should use the same one all througout your text. For pictures, I also use the past tense, because you've already seen it.