Thumbnail and Preview File from Picture Library Programmatically

Get Thumbnail File:

string urlthumbnail = oitem["EncodedAbsThumbnailUrl"].ToString();

Above will return thumbnail view url

SPFile filethumbnail = olist.RootFolder.SubFolders["_t"].Files[urlthumbnail];

Above will return file object and you can get absolute url

you can get absolute url of file 

string absUrl = Convert.ToString(oItem[SPBuiltInFieldId.EncodedAbsUrl]);

Get Preview File:

string urlpreview = urlthumbnail.Replace("_t", "_w");

SPFile filepreview = olist.RootFolder.SubFolders["_w"].Files[urlpreview];