Format string, integer with leading zeros

Use the format string "img_%03d.jpg" to get decimal numbers with three digits and leading zeros.

For posterity: this works with decimal numbers.

NSString *nmbrStr = @"0033620340000" ;
NSDecimalNumber *theNum = [[NSDecimalNumber decimalNumberWithString:nmbrStr]decimalNumberByAdding: [NSDecimalNumber one]] ;     
NSString *fmtStr = [NSString stringWithFormat:@"%012.0F",[theNum doubleValue]] ;

Though this information is hard to find, it is actually documented here in the second paragraph under Formatting Basics. Look for the % character.

https://developer.apple.com/library/mac/documentation/Cocoa/Conceptual/Strings/Articles/FormatStrings.html