Let me get this clear. You made the infographic on a website that requires you to pay for a higher-resolution download of the infographic right? If so, there's nothing you can do but choose another platform or simply pay the platform you used. The platform you used simply gave you a low-res version of the infographic you made, or you probably simply did a screenshot and removed the watermark.

This is not Imgur's fault. If the infographic image you have has decent resolution, then it would've still looked fine when you uploaded it on Imgur. Afaik Imgur doesn't compress or reduce the resolution of images uploaded on their site.