Basically, you have to draw the characters yourself in almost any image editing software. For Greek, you can save time by copying most characters from other alphabets like Latin and Cyrillic (Russian, Ukrainian) and only draw the missing ones, likely these: Δ Θ Ξ Σ Ψ Ω. If for some reason you can't or don't want to do it, I can do it - just let me know.