1.
If you want to know how to display costmized image on your arduino board, here is a project done by Arnov Sharma.

This blog will explain how to improve the quality of display via AdobePhotoshop.

2.
The OLED I bought can only display 2 color and cannot change its brightness. When displaying images like this with gradually changing color:

I converted it into C arrarys by using this online converter (Color Format may choose "Indexed 2 Colors"):

The final image on OLED screen:

For more complexed images, even harder to recognize: