If your photo looks normal in your gallery but turns dark when you try to post it to Instagram, you might be hitting a common HDR bug.
To achieve this look, reduce exposure and shadows while slightly bumping up the contrast to make the subject pop. 2. The "Text Overlay" Post Dark image
Many users on Reddit suggest taking a screenshot of the photo and posting that instead to avoid file-type conversion issues. 4. Creating Dark Images from Prompts If your photo looks normal in your gallery