MacBook – MBP Retina 13″, very contrast images stays on the screen

This is almost certainly a hardware problem. To determine whether it's the screen or the graphics card/motherboard you can plug in an external monitor. If you get the same problem on the external monitor then it's likely a fault with the graphics card/motherboard, if you don't get the same issue on the external screen then there's a problem with your MacBook's display.

Since it's an early 2011 model it should still be in warranty. I'd bring it to an Apple store and have it checked out for repair or replacement. On the plus side, you may get a brand new MacBook out of it.

I'm afraid that there really isn't a way to do it. I've been running into the same issue. The effective resolution at 1440x900 is actually 2880x1800 pixelwise- so the reason that you see crisp text is because you are looking at an image with 2880 x1800 resolution on a capable display. Because scaling down usually involves smoothing, the images will look terrible in email /on other peoples computers.
One solution is to download a program such as SwitchResX that allows access to "non-HDPI" resolutions. This would allow you to set the display to a true 1440x900, take the screen shot, and switch it back. Best of luck!