I agree with that, that one is great, but the others aren't quite as good. Keep making them though, it's something that gets easier as you go along. I'm not naturally good at comedy, but with a lot of practice I almost don't suck anymore!
The only advice I can give is don't try and make comics cold, if inspiration doesn't come to you then look for things to inspire you, I visit message boards and make comics about the people I meet there as well as the place where I work, stuff like that. I've never been able to just sit down and make a funny comic. Also spend some time in the forums if you want people to notice you.
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Dinosaurs had eggs bro, the chicken came way later.