Personally I just prefer to avoid the debate and call it “GNU IMP” v(._. )v
I think they could also get away with "gImp"
Or even, take after GIF and pronounce it with a "J" sound, like "JIMP"
the g in gif stands for graphics, so i could not accept jif as pronunciation and the creators chose it deliberately to match the jif peanutbutter brand, not because there was any logic to it. jimp does not match anything, except chimp maybe.