At the current time, Photos take up 24 characters and Links take up 23 characters, eliminating 47 available characters when both are included in a tweet. Twitter is reportedly introducing the change to give Twitter users "more flexibility" after exploring methods that would allow users to include more text in tweets.
Twitter has made efforts to give users additional characters to work with in the past, implementing tweet quoting features, using link shorteners, and removing character limits from Direct Messages.