base64标准格式转image：
NSData *imageData = [[NSData alloc] initWithBase64EncodedString:base64Str options:NSDataBase64DecodingIgnoreUnknownCharacters];
UIImage *image = [UIImage imageWithData:imageData];
base64字符串有图片前缀（前缀类似：data:image/jpeg;base64,xxxxxxx）转image：
NSURL *baseImageUrl = [NSURL URLWithString:base64Str];
NSData *imageData = [NSData dataWithContentsOfURL:baseImageUrl];
UIImage *image = [UIImage imageWithData:imageData];
