I don't know how to do it using IMG Editor but try this
Download txdworkshop.
https://www.gtagarage.com/mods/show.php?id=8320
Open txdworkshop. Click on "Open IMG" to the left.
Open your gta folder and load the gta3 file inside your model folder.
Type sniper.txd in the box.
Click on "SNIPERcrosshair" and click on "import" button at the top, then find your crosshair you want, load it and click "Save TXD"