Saving a Render image in high quality?

When I render my image in blender (F12), it looks nice, but when I save the image (F3). I get poor results. I set the quality at 100%, so I don’t understand why it seems that it is compressing it (the colors look like wave of the same color). I have tried Png and Bmp, which I understand that those are uncompressed formats.

What can I do to increase the quality of the image?

Can you post your saved rendered image as well as a screenshot to compare it to so that we can see what’s happening?

I used Gimp and it worked perfect. Thanks anyway!