If you want to add a icon image into your menu which its before of the menu list or after of the menu list. Simply you can use this Plugin Menu Image. It will be exact helpful to you what you expect.

Adds a field to load the image during a menu item and displays the image within the link within the menu before the text.
Now you’ll be able to transfer the second image and set to the mouse over/out impact.
And conjointly modification position of title or hide title if want.
And… load pictures compromise uploader!

After installed the plugin., You just move to Menu section.,

Appearance > Menus.,

You can just check  menu section where right side of the page while dropping a menu, there is part of selecting the menu images for responsible menu or custom link.

In this Plugin, Menu Image where has a option to fix a size of the image or icon.

Finally, the result will gonna rock !!