Many forums have specific size limits for avatars (often around 150x150), so your 1500x896 image is better suited as a discussion post image or banner rather than a profile icon. New Facebook Avatar Post Option

If you are on a mobile device, use Galaxy Avatar in Settings to create a character from a photo and export it to use in your custom-sized post. Posting Tips for Communities

If you want to use a personalized avatar directly in a Facebook post:

Scroll down the post options until you see "Avatar".

Pick a pose or background that fits your 1500x896 theme. You can also edit your avatar's appearance if needed.

If you are posting to a specific "Avatar" (movie or show) fan community like r/Avatar on Reddit :

Ensure your post is civil and avoids "hate or witch-hunting".