You have a few options... Paint.NET and GIMP are both free image editors you can use, they have their own merits, so use whichever you feel most comfortable with. Inkscape is a free vector graphic editor. While the final image would be a vector graphic, you may find the ability to scale and edit the contents of the blind without any loss of quality useful. Photoshop is obviously a paid program, but it rolls the features of the 3 programs I listed, along with many more, into one program.
I use all 3 of the free options for various different image editing purposes and they have their individual merits.
One important thing to note is that you must save rollblind images as 32-bit image files if they are to roll smoothly when on the bus. That's no so important for rollblind-based LED systems because they don't roll, but for default blinds it is.