To change a toolbar button image in Excel, first open the Excel file. Then, click on the "Tools" menu and select "Customize." In the "Customize" dialog box, click on the "Commands" tab. Next, scroll down the list of commands until you find the command that you want to change. For example, if you want to change the "Paste" button, find the "Paste" command in the list. Then, click on the "Paste" command and drag it to the toolbar. The "Paste" button will now be added to the toolbar.
Now that the "Paste" button has been added to the toolbar, you can change its image. To do this, click on the "Tools" menu again and select "Options." In the "Options" dialog box, select the "View" tab. Then, scroll down to the "Show:" option and select "Toolbar Buttons." This will display all of the images for the toolbar buttons. Find the image that you want to use for the "Paste" button and click on it. Then, click on the "OK" button.
The image for the "Paste" button will now be changed. You can repeat these steps to change the images for other toolbar buttons as well.