What is the resolution that you have set for the image of the infographic. As if it is set to let's say 72 it will look horrible. I always save images as PNG or GIF with resolutions of 300 dpi. But I also wonder if the platform your webpage is on compresses imagery. This could also be an issue as well too. For example, Facebook used to really compress images so they could load up quicker world wide as not all countries have high speed internet. Now, Facebook does not compress things as badly as they did 5 years ago.... but it could be that the platform you use to publish your website could be compressing images as well too. Just a theory...