Why do you add es to the end of the word volcanoes and only s to the word photos?

There is no doubt about it; English spelling can be a nightmare, even for native speakers. As it turns out, photo is an abbreviated form of photograph, the actual Word implied in 'photo'. That's how I remember that photo is made plural by just adding s, but that isn't really the whole story. This is a case where you have to get used to the many exceptions to the general rule to add es to Words ending in o to make them plural. Check the link below; it gives quite a few examples of the different ways to make Words plural. As you read more and more, the rules and the exceptions will become second nature to you.