image2cpp is a simple tool to change images into byte arrays to use with (monochrome) displays suchs as OLEDs, like those from Adafruit or Sparkfun. While searching for a way to generate these arrays, I mostly found links to a piece of Windows software. Both the flakey results and the hassle of having to boot a virtual machine just to convert an image lead to me writing this pure html + javascript solution.
Alternatively you can also enter a byte array as input to turn it back into an image. This might be useful for debugging, or when you want to write the byte array yourself. I don't know.
You don't need any special dependencies / internet connection; all the necessary parts sit in a single .html file. So just open this index.html page in a (recent) browser to run the tool. Or you can use the online version at: http://javl.github.io/image2cpp/
You can find a simple Arduino example sketch over here in the repository.
I wrote the code with my 128x64 pixel monochrome OLED display in mind, but it should work with most similar displays. You might need to change some export settings; those are explained in the tool.
Initial code by javl, with aditional code by wiredolphin, davidalim and whoisnian. The example sketch was made by Adafruit.