更改NSURL

NSString *string = [[jsonPost objectAtIndex:countPage] objectForKey:@\"excerpt\"]];
NSArray *array = [string componentsSeparatedByString:@\".gif\"]; //separated by dot
NSString *editedString = [NSString stringWithFormat:@\"%@_r.gif\",[array objectAtIndex:0]];
NSURL *url = [NSURL urlWithString:editedString];

NSRange range;
range.location = 0;//starting from the first character
range.length = string.length - 4;//excluding the last 4 characters
//of course you have to make sure the .extension part is 4 characters long(at least fixed length), not like .torrent or .rmvb
NSString *newString = [urlString substringWithRange:range];//this should give you the url part without the file extension

//then append the newString with something and make your url with this string