cs parameter specifies the color space of the output image. Changing or removing the color space metadata can help decrease file size, especially for smaller images such as thumbnails where it's a larger percentage of the total file size.
cs has several possible values that allow for different tradeoffs between color fidelity and file size. For higher fidelity,
srgb (the default) will set the image to the sRGB color space, which is the Internet standard.
adobergb1998 refers to the Adobe RGB (1998) color space, which provides accurate color reproduction from screen to print.
For better file size savings,
tinysrgb reduces the color space metadata but may cause a slight shift in color values, and
strip removes it completely for maximum size reduction.
See the API specification for more information about this parameter's value ranges and format.