Imgr is a free color tool that allows you to upload an image and it will automatically grab all colors from that image and present to you in a hexadecimal format.
Along with the output, you can also see a preview of your uploaded image. To view the hex code of a color, just hover your mouse over it. If you want, you can also download a sample of the color palette.
Imgr supports JPG, GIF and PNG format images. It does not seem to have any size limitation for uploading images.
Imgr is a simple and easy to use tool but with great use. No registration is required to use it and is totally free.