Hi,

First of all, I would suggest you to create document structure in your content tree in CMS Desk. For example:

- root
- Home (CMS.MenuItem)
- Folder(CMS.Folder)
-- File1 (CMS.File)
-- File2 (CMS.File)
-- File3 (CMS.File)

Then you can place repeater web part for example on your master page (root) in design mode in CMS Desk. You can ten configure it in following way:

Path: /Folder/%
Document types: CMS.File
Transformation: CMS.File.ImageGallery_thumbnails200
Order by: NEWID()
Select Top N documents: 1

In this way, your repeater web part should display random image from folder section where your files are stored. This is just an example, so you might need to adjust some settings by yourself.

Best Regards,
Miroslav Remias.