GIMP Version: 2.8.14
Operating System: Windows
GIMP Experience: New User

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How to change image 24bit rgb to 8bit rgb on gimp ?


i tried to find menu but not found, could you please help me ?

You need to change to indexed mode. Image ->Mode -> Indexed.
Then export your image in png format.



depending on image, choose dithering

Proof.

Using XnView to show the image properties before/after

When i tried to convert image using gimp. result 8 bit rgb image not beautiful as same as convert via https://tinypng.com/

how can i do ?


original image 24 bit rgb : http://image.free.in.th/v/2013/if/151212024837.png

image 8 bit rgb convert by gimp : http://image.free.in.th/v/2013/ir/151212024957.png

image 8 bit rgb convert by tinypng.com : http://image.free.in.th/v/2013/ig/151212025126.png


for tinypng.com tell me

How does it work?
Excellent question! When you upload a PNG (Portable Network Graphics) file, similar colors in your image are combined. This technique is called “quantization”. By reducing the number of colors, 24-bit PNG files can be converted to much smaller 8-bit indexed color images. All unnecessary metadata is stripped too. The result: better PNG files with 100% support for transparency. Have your cake and eat it too!

hmm.. life is too short for your image server


However, with patience I did get the smaller images and fortunately the subject is a well known wallpaper.

You did not read the line about dithering, depending on image.



In addition, I can duplicate your very banded gimp indexed image only by reducing the number of colors (to about 200) in the "convert-to-indexed" dialog.

Using a corner to save some large images.

this is your image from Gimp, very definite banding


This is mine from Gimp, 256 colors, still some banding but not as much.


Which is why you should be using one of the dithering options. This with reduced bleed dither.


You need to reduce the filesize?
Use a utility such as pngcrush http://pmt.sourceforge.net/pngcrush/