If you are looking at the same library, where the screen buffer memory is defined in the .cpp file:
static uint8_t buffer[SSD1306_LCDHEIGHT * SSD1306_LCDWIDTH / 8]
You will find that the buffer isn't defined as all zeros, with each hex digit coding a row of our pixels in binary. So setting it to blank is easy, just replace everything with zeros.
If you want to make it show a custom image you will need to do one of:
Find some code intended for graphic LCDs that can spit out bytes to paste in
Roll your own in programing language of your choice to edit and/or convert an existing BMP file by clipping it's headers
Convert those 0x00s to b00000000 and draft by hand in the text editor swaping 0s to 1s, which is how I've done it for LED matrixs but isn't fun.
