This project provides the tools needed for compressing/decompressing bmp for arduino projects
The bmp must be transformed into an array of bytes. "LCD Assistant" from http://en.radzio.dxp.pl/bitmap_converter/ is a great tool that you can use to convert the bmp into an array.
The array can then be compressed using the simple algorithm posted here. A working version can be found at this link:https://jsfiddle.net/sergiureznicencu/cfLrdufn/ .
On the arduino side, the provided function(drawCompressedBitmap_ver1) must be used for decompressing the array and printing it onto the display. This mechanism can be used when memory is very small but processing time is not as limited.