As title asks. I'm new to this website, I don't know how image uploading works or if I can do so for another person's game.

I want to share fanart for Juice Galaxy.

You can! Use the "add image" option on the top of the post-writing menu/window. (It may be the case that it only works for replies, but I highly doubt that.)

For example, I'll post a very old meme I made for the game below:

Also, know that Juice Galaxy has both a Discord Server and a Subreddit (linked in the main page for this game), which might be more advantageous/get a wider audience than posting to this forum. But in short, yes, you certainly can post images here. Whether or not Fish would prefer that people post fanart here or elsewhere is another question, though, and I'm not qualified to answer it. (I doubt that he'd mind either way.)