First of all, download the sniper crosshair of your choice.
Then open TXD Workshop, and click "OPEN IMG".
Navigate to your models folder inside GTA Directory.
Open gta3.img.
Search for sniper.txd and open it. After that, look for the sniper crosshair texture (default name 'SNIPERcrosshair') and click into it.
Click on "Import" and search for the image you downloaded and replace it.
When you finish, click "SAVE TXD".
There you go, you got your new sniper crosshair. If you have any troubles regarding this, message me or post here and i'll try to help you further.