I render large, sometimes over 5000 pixels and up, and save as a png.  Do my postwork and save as I go- as a png.  When I finally save the piece to post, I save as a jpg- lower the quality to hit below the 512 kb and make sure the size is at the max that's listed in the galleries to post.

When you save and work the render as a png file rather than a jpg, you don't lose data in the picture as you go.  I've tried to upload larger sized pieces than the max setting, but it won't let me do it.  Not sure if anyone else has.