Unsplash is a source for freely usable images. Unsplash has it's own Unsplash website where you can download images to use in your work. Please read the Unsplash licence agreement to see what you are allowed to do with the images and what not.
If you wish you can hide the Unsplash libraries altogether (disable the Unsplash feature) for your Office 365 tenant. Please contact us so that we can adjust this setting in your Content Chooser configuration. We are working on an officeatwork Admin Center app that will allow you to make these changes for your tenant yourself. The experience will be similar to the Office 365 Admin Center.