The answers depend on what has been said before. If you already mentioned that your friend was going to send some pictures, you would introduce them as "the pictures". You would speak of them as "the pictures", the specific ones you mentioned before. If you were talking about unrelated things and then, casually, just happened to take out some pictures to show, you would say these are "pictures". You would speak of them as "pictures" because they are not specific to your previous conversation. I hope this helps

We often say "some pictures" to distinguish the difference already mentioned. Whether you knew they were coming or had just arrived as discussed and explained in the answer. "these are (some) pictures my friend sent me From Canada" versus "these are the pictures my friend sent me from Canada"

Thank you for a clear explanation. The sentence was from a speech and a person was going to talk about the pictures. So the person didn’t mention the pictures. So the sentence was correct.

You would use "the" if the pictures had been talked about before. Since it sounds like they're just now being introduced, "the" wouldn't make sense because the reader wouldn't have known about them.