Can I upload an image to the 'Thank You' page?

I don't have a URL for it - it's simply saved on my desktop.

No, Our custom thank you page " Insert Image Tool " does not currently have an upload image option. It only currently accepts url links to your images. However, you can work around that problem if you don't have a url for your image by uploading it into our servers then just use the generated url. Alternatively, you could use a third-party image hosting service like Imgur.