Hi, mathangi. If you record a macro using Word's macro recorder, you are given the option to assign a button to the Quick Access Toolbar (QAT), and you can choose from a large number of button icons. However, I don't beleive Word 2007 allows creation of custom icons. It does allow renaming of the macro in the same dialogue which allows choosing a button icon.

If you have created your macro otherwise, go to your Word Options/Customize and select Macros from the drop-down list that reads "Popular Commands" by default. Select a macro, click Add to add it to the QAT. Select it in the right hand panel, click on Modify below. and rename and/or choose a button icon. The name will show when hovering your mouse over the button on the QAT.

Best,
Philip