How to convert a bitmap image 8bpp to 24bpp?

Could you tell me how to do that? How to convert a RGB value of 8bpp to 24bpp? (like pic bellow)

If I put the same RGB value of 8bpp in 24bpp, i look like the picture bellow.

How to make it like:

PS: English's not my primary language, please ignore my mistake :)

Answer 1

[Guillaume]

The simple answer is you can't, the conversion from 24-bit to 8-bit is lossy, there's no way you can get back the information lost.

[Doan Vu]

I mean have anyway to make 24bpp look like the last picture? Someways like smooth or something like that?

[Doan Vu]

anybody help me?