There is a small army of tools to help you with that. Some are paid, some are free: TweetDeck, SocialOomph, Buffer, Hootsuite, Twuffer, Twittimer, FutureTweets, Sprout Social, LaterBro etc. Not all of these support image tweets, but you can check them out or just make a Google search for "schedule tweets" and select the best tool for your needs.