Can we modify the default templates ( Change the default images,header files etc) and redistrubute them freely ? Where can i find such templates other than the default templates ?
SMF files cannot be distributed. It is against the licence.
themes.simplemachines.org will work.
hmm.. So all the templates found at themes.simplemachines.org are free to modify and redistrubute ? Do i need to get some sort of permission from the original creator ?
Read all licenses that come with themes. You must comply with those licenses. You must get permission from the creator to modify/redistribute/use in a custom theme.
All themes and files are free to modify, but redistribution is generally forbidden. In rare cases you will find it free, but usually you should ask.
Is there any backbone template files provided by simplemachine that we can use freely and redistribute them ?
You can base your theme off the default SMF theme.