To texture your flames just buy a Flame Particle Texture (FPT), get a texture you want for your flame, invert the colors and upload it to your FPT. Every particle of your flame will be changed to this texture.

If you just want the picture to appear once instead of alot of times, you'll need to buy or make a stable flame ,these are flames with 1 particle that doesn't move, making the FPT all you see.