Probably because:
1. You saved it in JPEG format.
Try and find the image again, or if you have to, re-do it, and save it again but when the 'Save As' box comes up, underneath 'File Name' select 'Save as Type' and select 24-color Bitmap. This may solve the problem.
2. The site resized the image.
If an image is too big, the site will automatically resize it so it fits comfortably onto the screen without overlapping. Usually, when you copy the blurred image back into Paint it goes back to normal size. This happened to me aswell. If you want to, re-do the image in a smaller size.
Hope this helps :-)

Yeah it could be the size issue, or you probably saved the file as jpeg. It always makes pictures fuzzy. If this is the case, I would re save it as png or gif. But if you do, you may have to resize the picture because the file gets bigger. Hope this helped!

You might be making the image too big. If its bigger than 2000 x 2000 or something, the site automatically resizes it, it drives me crazy. The other blank render probably came out fine because it wasnt over 2000 x 2000. If its not this then I'm not sure..