Often a picture is part of a larger file that is usually edited in some other major mode. Most keys do the same thing in Picture mode that they usually do, but do it in a quarter-plane style. There was no need to change C-a , as the choice of screen model does not affect beginnings of lines. Self-inserting characters replace existing text, column by column, rather than pushing existing text to the right. RET runs picture-newline , which just moves to the beginning of the following line so that new text will replace that line.
Text is erased instead of deleted and killed.
To do actual insertion, you must use special commands. This is useful for drawing lines in the buffer. Two motion commands move based on the current Picture insertion direction. Two kinds of tab-like action are provided in Picture mode. Without arguments it just moves point, but with a numeric argument it clears the text that it moves over.