Open your htc folder.
Open the AH2 folder
Open the 'sights' folder
Copy and paste the 'default' gunsight, it will show 'Copy of default' so rename it to mydefault (or whatever)
Right click 'mydefault' and open it with MSPaint
Draw a big honking black X across the bitmap
Save the file
Launch AH2 - Offline
Open Setup/Preferences/Gunsights
Scroll down the airplane list and highlight your test airplane by clicking it
Scroll down the sight list untill you see your new sight, 'mydefault' and highlight it
Click the button 'Set [airplane] position [position] to [mydefault]', then click OK
Go to hanger and select the test airplane
Deploy and look at the gunsight
Note that if the X you made touches any of the pixels along the border of your bitmap, then those pixels will smear beyond the sight and look terrible.
After you build your new sight, be sure to use .target offline and sight it in, see if you need to shift it around to center on the bullet pattern.
Hope this helps to put you on the right track.