Edit (again):Ugh, minor problem. Anyone know how to script in the ability to take screenshots? I suppose I could just paste from clipboard, never mind
Look for a "screenshots" folder, they should be stored in there. Or, just do a general search for "screen*.*" and see if the newest ones are the ones you took.
First, the image you posted isn't a PNG...it's a BMP file with the .png extension. Second, a JPG with 60% compression (Going by the IrfanView slider) should be decent enough, and makes the pic take up less than 5kb. You can probably vary it some for the ship, overall, I'd say that 5-10 KB for a preview pic is just fine.
There isn't a way to directly store a screenshot via script, but if searching doesn't help turn up the old screenshots, you could try moving the script to a different screen. Changing "$State: GS_STATE_INITIAL_PLAYER_SELECT" to "$State: GS_STATE_TECH_MENU" will replace the techlab with the ship viewer, and by that point game screenshots would definitely be working.