I have tried this on BOSS v2.0 (Anant) Linux’s Gnome desktop.
Find yourself the icon you want to use, use any image editor of your choice to resize the image to 22×22 pixels.
Look for start-here.png in:
rename it to start-here.png.old for backup.
paste your icon there and rename it as start-here.png and reboot.
This works on BOSS Linux running Gnome 2.18.2
For other distributions(Ubuntu etc) try searching for filename
<i> start-here </i> (Places>Search for files, be sure to select Filesystem). You’ll find many instances of the same file of different sizes. You can then change the icons by trial & error, be sure to keep copies of the original icons just in case something goes wrong.
A similar how-to for Ubuntu can be found here.