Is there any way to delete/remove files/images that reside in a separate shared

You might be able to use this Shared Component if you're using an on-premise Appian install.

forum.appian.com/.../summary

You could move the document into Appian (removing it from the server) and delete it or modify the plugin code to allow for deletion.

thanks. forgot to mention I am looking option for cloud instance, Any suggestion for that?

Check out the cloud approved SFTP Smart Services Plug-in (possibly 'Get Remote Directory Contents Over SFTP'), but I'm not sure if it'll remove the document from the File System after sending it.

forum.appian.com/.../summary

Alternatively, you could setup a web service on your network that Appian could ping periodically when the files need to be removed.

forum.appian.com/.../HTTP_Query_Smart_Service.html
forum.appian.com/.../Call_Web_Service_Smart_Service.html