If you receive a message with a media file link, you can download the media file using the method DownloadMedia.
TsgcWhatsapp_Client oClient = new TsgcWhatsapp_Client();
oClient->WhatsappOptions->PhoneNumberId = "107809351952205";
oClient->WhatsappOptions->Token = "EAAO4OpgZAs98BAGj3nCFGr...ZB2t8mmLB2LRXJkte2Y5PMNh2";
oClient->DownloadMedia("38923878928822", "c:\whatsapp\media\image.png");
To delete a previously uploaded media file, just call DeleteMedia and pass the object id as argument.