When you hit post and it doesn't do anything, that's a good indicator its too big. If you have a fairly modern phone the pics will be too big by default unless you mess with the camera settings.
What a lot of people do is take a screenshot of their pic on their phone and post that, then there's no chance for secret metadata to mine out of the pic (GPS Coordinates)
Do you know how to take a screenshot?
On my galaxy S10 its either by doing a palm swipe across the screen, or hitting the volume down button and power button at the same time.
Not all phones have the same way to take a screenshot, so you'll need to duck duck go (DDG) search how to do that.
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