If your organization does not supply a cover photo when submitting your profile a stock default image supplied by Communities Foundation of Texas is used.
You can change this image by logging into your organization's profile and selecting the "Edit Profile" button found on the right hand side just below the Users field.
You can find the cover photo field under "Organization Information" as shown below.
The preferred file type is PNG, but jpeg, jpg, and gif are accepted as well. The file size should be at 1140x300 for best results.