(Well it had a picture till they limited attachments to 30kb. what the heck. I'll link one in a sec) This image is shown at 200% to show it clearly. it's not to scale.
Ok. this shows how a sprite is made. This is the sprite file for the set 2 of the machinegun crosshair. It's labled to show you how sprites and the text files relate to each other. But I'm only going to explain the 800x600 one(shown in green).
So looking at the picture. You see that it starts at 40,0 and has a length of 50. It's as simple as that.
If you want to add a crosshair for aliens that didn't have a crosshair in 3.1. you'd have to change the first line to 27(or how many lines after the first line there are) so it knows how many lines to read. The crosshair_0 entry isn't there for some weapons since in 3.1 there weren't any for them.
So you'd use crosshair_0 320 for all of the crosshairs from 3.1
This will make it use that crosshair as crosshair 0 for any resolution since the 3.1 crosshairs did not use specific entries for different resolutions.
DO NOT EDIT YOUR OLD 3.1 TEXT FILES. CHANGE THE LINES IN THE NEW 3.2 ONES. weapon_weaponname.txt are located in your sprites folder. Edit them but be sure to back up your original just in case you want to change it back.