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The uncertainty about the correct spelling is caused by this:
English nouns ending in "o" such as potato, tomato, mosquito etc. have the "- es" ending in their plural form (potatoes). That is why one could reasonably expect also the word "photo" to have this kind of plural form as well, but it is not the case, for one reason or another.

Peter

Thanks, Peter, very clear explanation! Now I can be sure.

Students: We have free audio pronunciation exercises.

I see so much of the possessive form incorrectly used when one wants to express the plural of a word ending in a vowel!

Yes...it makes sence thank you...
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