Slack for Windows Phone may still be in development, but they are a myriad of different third party clients in the store that offer this functionality.
One of the latest apps (a universal Windows 1o client app) is called Just Chat. It is a sleek, well designed app that offers the basic Slack functionality including, pasting of images and quite obviously chat.
It offers sync across all your devices, meaning that if you sign in on one you are signed in on all. It also offers an optional dark theme, but this doesn’t seem to persist across restarts.
The app is available for Windows 10 and Windows 10 mobile right now for free and is a good stop gap while the official Slack Client is being prepared.
Download it from the source link below.