You can add a logo image on your Online Donate Button to enhance the branding of the donation window.  All you need to do is the following:

1. Login to your FirstGiving account.
2. Click on the Widgets tab
3. Click Make a Button
4. Click on Upload Logo
5. Find your logo on your computer and click Save
6. Save and you are done! The changes should now be reflected in the donation process.

The logo will resize to 80x80 pixels to fit in the location, so it's helpful to use an image that is already small, rather than having a huge picture shrink down to 80x80 size.  Also, since the logo space is square, it's best to use a square or squarish image so that it doesn't wind up distorted.