This is a bit rough but I use it to make bitmaps I can embed into my C code which I can then use as displayable icons.

To make the icons I use Gimp to create a 28x28 pixel image then export it as a .h file. This .h file cannot be directly used because it is an awkward format. So I pass it through a transformation I wrote in C++.

Because this is just a utility, and I only have 10 icons I just paste the output from Gimp into a static char * in MakeIcons.cpp. You'll find then all near the top. Then in the main method I process each one. For each of them I create an Icon object and then call the Icon's draw() method. That writes out the bitmap in a useable format. The smart stuff in this is in that draw() method where it does a lot of bit shifting etc to get the 16 bit output right. And then it prints it out. The result looks like this:

 static uint16_t myicon[] PROGMEM = {
   0x0000,0x0000,0x0000,0x0000,0x0841,0xffdf,0xffdf,0xffdf,0xffdf,0xffdf,0x0841,0x0000,0x0000,0x0000,0x0000,
    0x0000,0x0000,0x0841,0xffdf,0xffdf,0xffdf,0xffdf,0xffdf,0xffdf,0x0000,0x0000,0x0000,0x0000,
    0x0000,0x0000,0x0000,0xf79e,0xffdf,0xfa8a,0xf8c3,0xf8c3,0xf8c3,0xf9c7,0xe410,0xf79e,0x0000,0x0000,0x0000,
    0x0000,0xf79e,0xffdf,0xfa08,0xf8c3,0xf8c3,0xf8c3,0xfa49,0xffdf,0xf79e,0x0841,0x0000,0x0000,
    ...

In my main project (the one that actually displays the icons) I paste that bitmap into the relevant cpp file and draw it with code like this (assume the *icon points to the 16 bit icon):

 int count = 0;
 for (unsigned int j = 0; j < size; j++) {
    for (unsigned int i = 0; i < size; i++) {
        uint16_t pixel = icon[count++];
        if (reversed) {
            pixel = ~pixel;
        }
        Graphics.drawPixel(x + i, y + j, pixel);
    }
 }

The size variable is almost always 28 because we started with a 28x28 pixel image. But you can change the size and you'll see I do this in the last bitmap which is a 14x14 image. The same code is used to display that icon, as long as I tell it the right size. I can also optionally reverse the image by setting a flag which just negates each pixel.

The project is in github.