It's pretty simple but I'd recommend using OpenIV for this, SparkIV resized images that are bigger than the original and the way they do it causes a loss in quality.
First make sure OpenIV is in edit mode (hit the big edit mode button in the top right corner). All that you have to do then is open the .wtd file of the gun you want the icon replaced, select the icon and then hit replace. Then select the custom icon you have.
If you're determained to stick with SparkIV though I'd imagine the process is pretty much identical (except for the edit mode, just make sure you save after replacing the files).
You can only do it one gun at a time, so you'll have to repeat this process for every gun you want the icon changed