I'm definitely not qualified at this time, but what particular Lisp is used? I just started reading SICP and I'm interested how Lisp's used in a web app environment. I think I've heard that Hacker News also uses a Lisp variant?
Ah, that might need clarifying. The Python picture is suggesting the language is for children. But the Arc image is because Arc itself is a young language, not because it's simplistic or childish. That said, I wouldn't recommend Arc, unless you specifically want to code for Hubski. Not only because it's young, but it's not widely used, and doesn't really offer any advantages over other LISPs. Scheme is clean and simple, but lacks real macros (which are a huge deal, really). Common Lisp is powerful, fast, and widely used; but necessarily complex. Clojure lets you integrate with the JVM, has good concurrency support, a compile-to-javascript variant, and is the current fad; but has painful tooling and installation, and the startup time of a snail riding a turtle riding a tectonic plate. I'd suggest picking whichever of those sounds the most appealing. They're by far the most widely used, and will be easier to find help and docs for. I'd also think about looking into Erlang and Scala. They're functional but not LISPs, and definitely worth the consideration.
I wouldn't start with CL, despite CL being my favorite. It's a lot to take in if it's your first exposure to lisp, and a lot of things only make sense if you know either their history or how CL implementations work. Scheme can be frustrating to do real work with, but you can pick it up in an afternoon (modulo call/cc, which won't make sense for a long time) and a little playing with it will tell you whether lisp is for you. There are many Scheme implementations with real macros, but hygenic macros are neat and worth learning too.
Right now I'm running through SICP with Scheme; I'll look into Common Lisp as well. Arc interests me purely from a webdev standpoint, unless the other Lisps also work well with the web? I've also heard of Clojure but I'm not too interested in compiling to either the JVM or Javascript. Erlang also interested me in the past but I'm not looking into it at the current moment. Maybe one day.
Clojure has good web libraries, and Clojurescript compile-to-js means you can write both the back- and front-end in Clojure. For what it's worth, Amazon's back end used to be C and Common Lisp, and they've recently posted openings for Clojure devs. I mean, even Scheme has web server libraries (and if you want something simple, it's not a bad choice). But if you want a LISP with extensive web frameworks and protocols, Clojure is really it. Arc just doesn't have the popularity—when you hit walls, it's going to be hard to find help.