If you are using Sharepoint 365:

According to Microsoft 

> In a recent update to Office 365 that is currently being deployed, the
> ability to automatically switch to new images is being deprecated for
> libraries that do not have a Content Delivery Network (CDN) enabled.
> This means that the AutoPlay functionality for existing websites will
> no longer function for Picture Library Slideshow web parts.

The full information from Microsoft can be found [Here][1]

To use the CDN together with the Picture Library Slideshow web part, your SharePoint Online tenant administrator must enable the CDN within your Office 365 tenancy. 


You can find the [steps to enable CDN within tenancy here][2]

But to help in case of link death here is the pertinent information

> Using Office 365 CDN in SharePoint Online
Here are the steps to get started with the Office 365 CDN.
> 
> Make sure that you are using the latest version of SharePoint Online
> Management Shell You can download SharePoint Online Management Shell
> from following link:
> https://www.microsoft.com/en-us/download/details.aspx?id=35588
> 
> Connect to your SharePoint Online tenant with PowerShell
> 
> Connect-SPOService -Url https://vesaj-admin.sharepoint.com
> 
> Get status of the CDN settings from tenant level
> 
> Get-SPOTenantCdnEnabled -CdnType Public / Private
> 
> Enable CDN in the tenant
> 
> Set-SPOTenantCdnEnabled -CdnType Public / Private ...
> 
> Public CDN enabled locations:
> */MASTERPAGE
> */STYLE LIBRARY
> 
> Disable CDN in the tenant
> 
> Set-SPOTenantCdnEnabled -CdnType Public / Private -Enable $false
> 
> Get default configuration of your public CDN settings
> 
> Get-SPOTenantCdnPolicies -CdnType Public ...
> 
>  Key Value   --- -----   IncludeFileExtensions
> CSS,EOT,GIF,ICO,JPEG,JPG,JS,MAP,PNG,SVG,TTF,WOFF 
> ExcludeRestrictedSiteClassifications   ExcludeIfNoScriptDisabled False
> 
> Add new CDN origin
> 
> Add-SPOTenantCdnOrigin -CdnType Public -OriginUrl sites/cdn/cdn ...
> 
> Public CDN enabled locations:
> */MASTERPAGE
> */STYLE LIBRARY SITES/CDN/CDN (configuration pending)
> 
> See the list of CDN origins
> 
> Get-SPOTenantCdnOrigins -CdnType Public ...
> 
> */MASTERPAGE
> */STYLE LIBRARY SITES/CDN/CDN
> 
> Remove CDN origin
> 
> Remove-SPOTenantCdnOrigin -CdnType Public -OriginUrl sites/cdn/cdn ...
> 
> Public CDN enabled locations:
> */MASTERPAGE
> */STYLE LIBRARY



  [1]: https://learn.microsoft.com/en-US/sharepoint/troubleshoot/web-parts/non-cdn-enabled-picture-library-slideshow-web-part-no-longer-switches-automatically-to-new-images
  [2]: https://web.archive.org/web/20180202220410/https://dev.office.com/blogs/general-availability-of-office-365-cdn