San Francisco - Yahoo! joined the fray over smartphone photo-sharing on Wednesday by making it more enticing for iPhone users to use its Flickr service amid a rift between Twitter and the popular photo app Instagram.Improvements to Flickr software tailored for iPhones and iPod touch devices included the addition of Instagram-style "filters" that let people imbue photos with artistic effects."We know that some of your best photo moments happen on the fly, so we've made it easier to get the perfect shot when inspiration hits," said a Flickr blog post by Markus Spiering."Once you get the shot, there's a built-in editor to quickly correct, crop, or enhance it with one of the new (high-resolution) filters."Pictures taken with iPhones or iPods can be shared at Facebook, Twitter, Tumblr and Flickr, or by email, the Yahoo-owned photo service said with a promise that photos will "look gorgeous no matter where they are viewed."Feud"We built the app with sharing in mind," Flickr vice president Brett Wayn said at the Yahoo! blog."We've worked hard with our partners to make sure your pictures are displayed beautifully on Twitter, Facebook, and Tumblr."The move came after Twitter added Instagram-style smartphone photo sharing features after the Facebook-owned service made it impossible for Internet users to integrate its images into tweets.Twitter said its partner Aviary is powering "filters" and other effects for images using the latest Twitter applications for Apple iPhones or smartphones running on Google-backed Android software.A social media feud between Twitter and Instagram escalated over the weekend as the popular smartphone photo-sharing service made it impossible for internet users to view its images in "tweeted" messages.Instagram, which has some 100 million users, is seeking to route photo viewers to its own website, where it has the potential to make money from ads or other mechanisms, instead of letting Twitter get the benefits.Previously, Instagram pictures shared in messages tweeted from smartphones could be viewed unaltered at Twitter.Instagram rose to stardom with the help of Twitter, but has distanced itself from the messaging service since being acquired by leading social network Facebook.Facebook completed its acquisition of Instagram in September. The original price was pegged at $1bn but the final value was less because of a decline in the social network's share price.A distinctive feature of Instagram is that it allows users sharing smartphone snaps to enhance them with image filters for artistic effects such as mimicking historic types of film.Yahoo! is out to adapt photo sharing and storage service Flickr to modern lifestyles as part of a focus by newly-minted chief executive Marissa Mayer on connecting with people on smartphones or tablets.