I found a way to change the Main Menu Icon in Gnome. It is a bit "round about", but it works. Also it uses the GUI instead of the terminal so an individual who does not know terminal commands can do it.
1. Download the custom Icon you want to your Home folder. I chose an Icon theme.
2. Go to the compressed file and right click on it and choose the "extract here" option.
3. Log out of your user account and log in as root. (I know this can be dangerous, and some of you are going NOOOO
4. Navigate to /usr/share/icons and leave that window open. In a seperate window, navigate to /home/(your user name)
5. Drag and drop the folder your extracted the icon theme into to your /usr/share/icons window.
6. Open the Icon Themes folder and look for an icon named start-here.png
That is the icon used by the main menu in Gnome. Once you find it, rename it to start-here2.png
7. Copy the icon you want to use into the folder where you found the Start-here icon, and rename it start-here.png
8. Log out of your root account, and log back into you normal account.
9. Go to your Appearance settings and choose customize. In the customize box, click on the Icon theme you downloaded, and close.
Now your Main Menu Icon is the one you downloaded.
I know that there is an easier way out there to do it, I could not figure it out. If you know, please post it here. Please bear in mind that this was posted by a Total Linux Noob, so be gentle with you comments.
Thank you and I hope this helps.