Nopcommerce do not have that support now, you can modify the source if you like to do so.
Find Nop.Services->Common -> PdfService.cs
Here is the code how logo works on invoice, you need to duplicate same thing in PrintPackagingSlipsToPdf method
public virtual void PrintOrdersToPdf(Stream stream, IList<Order> orders, int languageId = 0)
{
...
#region Header
//logo
var logoPicture = _pictureService.GetPictureById(_pdfSettings.LogoPictureId);
var logoExists = logoPicture != null;
//header
var headerTable = new PdfPTable(logoExists ? 2 : 1);
headerTable.WidthPercentage = 100f;
if (logoExists)
headerTable.SetWidths(new[] { 50, 50 });
//logo
if (logoExists)
{
var logoFilePath = _pictureService.GetThumbLocalPath(logoPicture, 0, false);
var cellLogo = new PdfPCell(Image.GetInstance(logoFilePath));
cellLogo.Border = Rectangle.NO_BORDER;
headerTable.AddCell(cellLogo);
}
//store info
var cell = new PdfPCell();
cell.Border = Rectangle.NO_BORDER;
cell.AddElement(new Paragraph(String.Format(_localizationService.GetResource("PDFInvoice.Order#", lang.Id), order.Id), titleFont));
var store = _storeService.GetStoreById(order.StoreId) ?? _storeContext.CurrentStore;
var anchor = new Anchor(store.Url.Trim(new char[] { '/' }), font);
anchor.Reference = store.Url;
cell.AddElement(new Paragraph(anchor));
cell.AddElement(new Paragraph(String.Format(_localizationService.GetResource("PDFInvoice.OrderDate", lang.Id), _dateTimeHelper.ConvertToUserTime(order.CreatedOnUtc, DateTimeKind.Utc).ToString("D", new CultureInfo(lang.LanguageCulture))), font));
headerTable.AddCell(cell);
doc.Add(headerTable);
#endregion
....
}