Not speaking as an expert in any capacity, but I learned quickly to not expect to bang out a masterpiece right out of the gate. Don't expect anything to go smoothly. You can be proud of your work, and should be, but don't stop learning. I'm always messing with lighting and props to try and bring it to the next level. But working with SFM shows you how much the textures and models are bugged in Team Fortress 2 and how they are repaired on Valve's side haphazardly, though I can't be too critical because I'm no expert at textures myself. Like, at all.
Still, You'd think if someone is getting paid for this and releasing it to the community to work with, it'd be a little easier to work with or at least not such a pain to fix.
If you are serious about doing quality posters, get familiar with YouTube, Facepunch, and Google. You're going to need them. A lot.