DefaultWebOptions.AllowPNG Property (Excel)
True if PNG (Portable Network Graphics) is allowed as an image format when you save documents as a Web page. False if PNG is not allowed as an output format. The default value is False. Read/write Boolean.
expression A variable that represents a DefaultWebOptions object.
If you save images in the PNG format as opposed to any other file format, you might improve the image quality or reduce the size of those image files, and therefore decrease the download time, assuming that the Web browsers you are targeting support the PNG format.
Alternatively, PNG can be enabled as the global default for the application for newly created documents.
Application.DefaultWebOptions.AllowPNG = True