The symbols in Project Designer is basically just bitmap (*.bmp) pictures. Attached you can find them so that you are able to modify them or get an idea of the sizes used if you wish to create your own.
Also, you can find the symbol for a TV and our Collabo, these can be put in the below file path and will be visible in the Uniform graphical elements on the Interface page.
Finally, there are pictures of some other devices, which are just BMP, simply just drag and drop them into your Interface view and if you wish to save them, then just right click and press save as template.
Symbols are looking best with straight lines or 45 degrees because of the low pixel density on the E-ink display. Symbols can easily be created in Windows Paint or similar applications.