If you can make it with GIMP, you could probably look into OpenGL shaders to do it directly yourself: the processing could be faster and would provide you with

If you can make it with GIMP, you could probably look into OpenGL shaders to do it directly yourself: the processing could be faster and would provide you with more flexibility and coding fun times. I don't know of server-side libraries for that, but on iOS we have MetalPetal which provides some filtering features like ones in Gimp/Photoshop. Have fun!