NSString* strName = [NSString stringWithFormat:@"img_00%d.jpg", pp];
and works fine, but if pp took the value of 10 for example I wish have img_010.jpg as result, and not img_0010.jpg... how can I format the string??
Use the format string
For posterity: this works with decimal numbers.
Though this information is hard to find, it is actually documented here in the second paragraph under Formatting Basics. Look for the % character.