There is a way in your PayPal account to change the graphic. When you are in your PayPal account and you choose to Create a PayPal button, there will be a Customize Button section. It will then have a dropdown, "Customize text or appearance". Click on that. Then there will be a radio button option for "Use your own button image'. choose that and then enter the URL where you graphic is.

I am not sure if the following will work as I have never tried it myself, but a possible alternative would be to copy the code for an existing PayPal button that you have. Then you'll see a tag that looks like this:
<input type="image" src="put your image url here"...

I don't know PayPal's internal workings so if they have some kind of mechanism to associate the image that you create using their method, the alternative method may not work. But if it does, it will be a lot quicker.

Best Regards,
Jim

Sorry that I can't help you out with this, but would you mind letting me know how to do this? I tried, but couldn't work it out, so I simply ended up creating a link that opened a new paypal donation page... Thank you!