Yes, you can upload your own assets. Use high resolution .png, .jpg or .jpeg files. Make sure they are images you have the rights to, and that they accurately depict your plates as well as your brand and style of service.

If you uploaded your logo in your profile “Brand”, it will be available for use in the Design portion of Sysco Studio, in the image gallery under “My Files”. 

You may also upload custom backgrounds directly into your Gallery / My Files, then use them as the “bottom layer” of your design.