The ship itself isn't actually a screencap - I downloaded a free legal copy of 3DS Max from Autodesk Education USA, took Flat_Eric's Voyager mesh, changed the registry textures and re-rendered the model with a STO wallpaper in the backdrop.
Go to DS9 system space and position your ship perpendicular to the sun near the wormhole. This will give you very good lighting of the ship's hull.
Open screenshot in photoshop, magic wand the ship out from the pitch black backdrop and insert into sci fi art background of your choice, or just leave the ship sitting in black so it matches the forum background. Sigs can be made in 5 mins flat if you know your way around photoshop.
** - Some of the sector space areas in Cardassian territory have awesome backdrops for that "Eye of Sauron" fire effect. Breen campaign mission sets also have awesome backdrops for certain maps.
STO screenshots on their own are also good enough to create sig pics directly from. Even on low end graphics cards you can just temporarily bump up the settings to max, add some JJtrek glare effects and viola, sig-worthy pic.
Since you are downsizing the picture from full resolution to 500x150 the downsizing plus sharpen +25% gives you some sort of automatic anti aliasing for a smooth picture.
OR ... MS paint if you want keep it really simple :p
actually in all seriousness, early on when I didn't know how to use certain functions in photoshop, I used MS paint to zoom in to remove individual pixels around images that I had. But it was very tedious.