One important images to put in your website (best in your ecommerce site footer) are the images of the credit card and payment methods that you accept. Following are some images and other sources that you can find them:

Visa Buttons

You can download a good number of marketing and promotional material from Visa website itself. Here are some other ones:

Master Card Buttons

MasterCard also has promotional material on their website and here are some more.

Discover Buttons

American Express Buttons

PayPal Buttons

You can download all PayPal buttons here.

Another great website for credit card images is here.