To upload the creative within Pexi you need a Pexi account and access to the correct advertiser. You can request a Pexi account via traffic@pexi.nl. When you have access to Pexi you can upload the banner as a zip (zip the folder, not the files). If you include the IAB size within the filename Pexi will automatically detect and adjust the upload settings.
The slider works as a gallery for four images. The user is shown four thumbnail images at the bottom of the creative to give them a small preview.