To answer your specific question first--most raw conversion software can produce a lower resolution jpeg file from a raw file. The DPP software that comes with your G10 will do the job (click Convert and Save under the file menu and then complete the dialog settings for the desired size). Adobe Camera Raw will also open files into Photoshop at various sizes which can be saved as jpegs.
With the G10, you already have the ability to shoot low resolution jpegs at any time should you find it useful to do so. However, shooting a lower resolution file by itself does not solve the problem of high ISO noise. If you shoot a high ISO image with the G10 (or any compact camera) there will be considerable noise. That noise can be corrected (to a degree) with the use of noise removal software (either in-camera software or software on your computer). Any noise reduction software will have the unfortunate side effect of destroying detail in the image (the more you reduce noise the more detail you will destroy) so that the image after noise removal will have no more detail than if you had shot a noise-free image at lower resolution. Some people might say that you should convert the file to a lower resolution file at that point, but I would prefer to leave it at the original size and simply print at a smaller size.
If you want to get the best possible high ISO shots from the G10, I would suggest shooting in raw, converting the file at full resolution with your preferred raw converter, and applying noise reduction with a program like Noiseware Pro.
Yes understood, but I wanna know if a software exist that can do for example "mean of 4 pixels" to obtain a lower resolution but higher quality picture (4 time the pixel surface) !
G10 can also produce awfull JPG picture, see below, but shooting RAW allows you to greatly improve quality, 2nd sample !