Spotting a few problems
I can clearly see the different fractals used here and they go together rather well, but what's with the glare I'm seeing on the red one? It looks extremely out of place here and shouldn't be there. also the rainbow ring was a pretty nice addition but the obviousness of it seemed extremely out of place. A fatter and more transparent ring would work better here.
Overall it's a pretty nice picture but wasn't done right in little places.
Just a suggestion, you might want to look into using apophysis. The fractal flame filter in GIMP is actually exactly the same as Apophysis but extremely limited in features. If you want to look into creating more complex fractal flames you will want to look into it.