/ic/ - Artwork/Critique

After I'm done drawing something digitally and have to change the size and resolution for upload, how do I avoid the image having sever aliasing issues? This happens regardless of the dpi and regardless of if I save it as a PNG or JPG file. I usually work over 4000-5000pxs both horizontally and vertically at 600 dpi, usually try to resize my image just over 1000 pxs both horizontally and vertically at around 300 dpi, regardless I have aliasing issues. Example attached.