I've had the same problem, but on another platform. Turned out, it's a problem with the site's compression algorithms. For reducing the size of your image, they compress a part of image data. So, the output image "weights" less, but the quality is reduced too. In that case, I would ask if moderators could allow you to increase the limit of such data. Also, you can make sure your images are good enough by processing them on an
image sharpening service. Unlikely it will solve the problem, but you can set aside doubts about your pictures.