Creating a custom logo or banner in Gimp is not that hard. If you are unfamiliar with Gimp, it can be scary, but this is a very useful and FREE tool that I use all the time to edit images. It's definitely worth investing some time in.
This skill is particularly useful if you want to make unique banners and logos. For example, everyone promoting WA has the same banners on their site. Get something a bit different, and customize it to match the theme of your site!
http://www.canstockphoto.com (paid images)
This training should take you through the whole process (simply), but there are many more options. Check out YouTube for lots of free gimp tutorials.