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Yahoo! joined the fray over smartphone photo-sharing 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.