This part is the most simple, If you want an img for the backgorund, Click slider until the "Image" is toggled. Otherwise, Keep the slider on the "Solid Color" Option.
Now, this is where it gets harder. To set a background image, you have to make it what is called a Data URI. This is due to the fact you can not upload images to Deskthing (i don't think). To do this go to this website (click here for site) and paste in your image. The site should output a code that is somthing like "data:image/jpeg;base64,(random letters)". Copy the code to your Clipboard. Then simpaly paste it into the textbox labeled "Background Image"
Setting a background color is as easy of choosing a color, getting its Hexadecimal Color Code (should look shomthing like this "#XXXXXX") and pasting it in the "Background Color" textbox. To get a color search it on any search engine, say somthing like "(color) Hexadecimal Color Code", and the code should be somewere.