RS232 is a hardware interface protocol and is not what we are talking about here.
The simple answer is that a UART can send any 8-bit value which means that any binary value can be sent.
However, if I was to give you a value, how would you know what it means?
You talk about pixels so presumably they some in a sequence of frames which is made up of a certain number of rows, each of which has a certain number of pixels. Unless you have some higher level protocol that says "I'm about to send you a new frame" or "This is the start of a line" or "There are 64 pixels following this" then you have no way of the receiving end getting into sync with the sender. The protocol could also be in hardware - "when I raise this I/O pin, the following character is the first of a new frame".
THAT is the protocol that is being talked about here. It can be whatever you want it to be but without it, whatever you send is just noise.
Susan