Have a submitted image from a sub domain, show up on the main domain?

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PHP

I have a subdomain with a "meme generator" on it. Then i have my main domain with a category "meme". I have two idea, on once the meme is generated, it automatically gets uploaded to the meme category, and/or there is a button that the users can press to upload it to the category.

I am lost on how to do this since once script is on a sub and one is on the main domain.