If you look at the "Sukena" sig around the area of the "k", you might notice that everything to the right of that point is a mirror of the left side of the image - it wasn't wide enough, but I was happy with it because I knew I'd be covering it with text. Is it noticable?
In the "Lightning" sig I had to rebuild the coastline of the Thundering Steppes at the top right of the image by hand as it was, again, slightly too short. I'm actually stood on the bowsprit of the ship at the docks in beta (hence the pink armour, ranger armour was briefly pink chainmail) which is why the winged skull mainsail and some of the rigging is sat in the background over my shoulder.
A lot of the work I put into creating sig's is taking the screenshot that will be transformed into the background, which can often lead to interesting way's of working with the game engine to create the effect I am after
I find taking them in as high a resolution as is managable is a good rule of thumb, as the resulting image is more versatile when you need to start scaling it down.
I'm looking forward to getting into Vanguard and making screenshots of my Kurashasa character - I am hoping they will have many fur patterns akin to EQ2's Kerra, and not only the "solid colours" we have seen so far in the Silky Venom E3 movie.