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Yes, I feel as though going this route will be a bit clearer. However, it's up to your final decision. I think if only one will count, then you may as well save the others for other contests/challenges.

Thanks.

You can add more photos to that post, but only the first one in your post will be your submission for the contest.

I've changed it into this ^
I hope that makes it a bit clearer. I'm no native English speaker 😉

Perfect, That's clear. I'll revise the rules in my template. Thanks.