image2cpp is a simple tool to change images into byte arrays (or your array back into an image) for use with Arduino and (monochrome) displays such as OLEDs. It was originally made to work with the Adafruit OLED library. An example sketch for Arduino and this library can be found here.
More info (and credits) can be found in the Github repository. This is also where you can report any issues you might come across.
This tool also works offline. Simply save this page to your computer and open the file in your browser.