Connected Stories NEXT has an Image Library which supports PNG and jpeg files and a Document Library for .MP4 videos.
HOW TO UPLOAD ASSETS
After creating your first story, choose one of these three icons to upload your first interactive video
...
, interactive image
...
, or interactive gallery
...
located at the right of your story editor.
Select 'Drag or Click Here to upload your video or image' located in properties on the right side of your screen.
...
This will bring you to our Asset Gallery. Now you can create a new folder for your campaign using the 'Add' button to store all your assets.
...
To upload an image or a video, click on the folder you just created and then select the
...
button again and choose 'upload file' from the dropdown menu.
You can either drag and drop files into the upload area or you can click the upload area to browse through your files and select the assets you want to upload.
To edit and manage assets, click on their three grey dots in the upper right-hand corner.
To bring your uploaded assets into your story, select the asset from your asset gallery
Please wait for transcoding while uploading a video. 
...