Please say if there are any special requirements or limitations for image files I want to use for brading my application. Size, colors, format, etc.
You can use any images, formats and colors that are supported by modern web browsers.
If you are talking about “branding” directory in themes, we use two *.png icons for login window and menu panel which you can replace in your theme extension. And you can use the different format of these images if you set the path to your images from Studio: Project properties - Branding menu.
What particular formats of images you are interested in?