Hello Sekicion. Many sites lowers the quality of picture such as Facebook and I'm not sure why. But if that happens on Deviantart which keep the hd quality there might be problem on how you save your file.

I usually work on A3 (international paper) size with 300 resolution. When I save files for uploading I lower the size to "800x1131 pixel" and resolution to "72" and save it as a "png" file(png is a good format for internet uploads).

Try this and if there is still a problem you may upload the the full resolution file (300 res) and size which that will make your file a lot bigger and clearer but sharing original files on the internet put your work in danger from "art theives" and such.