Images greater than 1920 x 1080 pixels are optimized twice, once to scale the image down to fit within the maximum image dimensions allowed within the platform (which is 1920 pixels for width and 1080 pixels for height).
Then a second time to optimize the image file size.
If the image uploaded to the platform has dimensions of 1920 x 1080 pixels or smaller the image will only be optimized once for file size.
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If you want to keep the most quality possible. Re-save the image outside of the platform so that its dimensions are at or under (both) 1920px for width and 1080px for height.
Save the image as a high quality .png.
.png files are lossless and we don't remove as much quality from the file when optimizing them.