Inkscape is a free, open source application via which you can create vectory graphics. You can also use Inkscape to create your facebook page main graphics such as the logo and the cover photo, maybe also some images to highlight and assign as album covers.
In order to create your first Facebook cover photo for your page, first open us inkscape :
Second, go to File-> Document Properties and assign the canvas size to match the Facebook cover photo size in pixels, as below :
Next, you can add a rectangle to your cover and some text, via the tools to your left, the mail tool bar of Inkscape as you can see in the next two images :
Remember that you can save this as png which is something Facebook can "understand" and also it would be wise to save this as "Inkscape SVG" which is a native form of Inkscape file type which you can open again later via Inkscape and be able to edit it.
Then, you can change your cover on your Facebook page :