, если вы не собираетесь анализировать html, тогда будут работать простые substringFromIndex и substringToIndex.
NSString *yourFullURL = fullHTMLString;
NSRange t = [yourFullURL rangeOfString:@"\"><img border=\"0\""];
//check range exists
if(t.location !=NSNotFound){
NSString *yourJPGURL = [yourFullURL substringToIndex:t.location];
}
// this will give you a new string up to the end of the URL//
// then repeat for the first part using a new NSRange for the beginning of
// the URL, use range.location+x (where x is the number of characters to add as NSRange
// returns the beginning of the string you search for.