If your intent, as described, is to teach vocabulary: it truly doesn't make any difference. Verb tense in online or printed photographic display is an element of grammar. When describing photos in spoken English we also vary in verb tense, "This is me playing baseball when I was twelve." "I played baseball when I was twelve."

If you are fluent in your students' native language, I suggest you explain the English grammatical issue in that language.

If you are not fluent in their native language and/or the students are children, I would not address the issue. Some aspects of English grammar are best left for advanced study or avoided althogether, articles for instance.