Much better than GIFs at expressing sentiment.
Free Slack plans only allow 10 integrations. So your admin may have to upgrade to a paid plan to add Vlipsy if integrations are maxed out.
My first inclination is to look for a Vlip to share on Slack. If the perfect one isn't available, and if I have a couple of minutes to spare, I might go to Vlipsy's website and create one. It's not that hard. Plan B is to use a GIF.