Maybe try removing read only from the proprieties of all GTA files. 
right click on your models folder and click Properties. Uncheck the box that says "Read-Only" and click OK. 
If that does not help....
Reinstall your GTA, make a backup of the files you usually modify for example gta3.img or player.img
Then uninstall IMGtool, and reinstall it following all of the directions.