using (var doc = PdfDocument.CreateNew(new PdfForms()))
{
    //Set default appearance. Helvetica and autosize is used by default
    if (!doc.FormFill.InterForm.HasDefaultAppearance())
        doc.FormFill.InterForm.SetDefaultAppearance();

    //Create a pushbutton field 
    var pushbuttonField = new PdfPushButtonField(doc.FormFill.InterForm);

	//Create a pushbutton control that uses default appearance.
	var page = doc.Pages.InsertPageAt(0, 500, 500);
    var icon = PdfBitmap.FromFile(@"e:\0\stamp.jpg");
    var btnRect = new FS_RECTF(10, page.Height - 10, 10 + icon.Width, page.Height - 10 - icon.Height);
    var pushbuttonCtrl = new PdfPushButtonControl(pushbuttonField, page, btnRect, "Button caption", null, null, null, null, null, 0, icon, FormHighlightingMode.Push, TextPositions.TEXTPOS_ICON);

    //Add the form field to the document.
    doc.FormFill.InterForm.Fields.Add(pushbuttonField);

    //Save the document
    doc.Save(@"e:\22\test.pdf", Patagames.Pdf.Enums.SaveFlags.NoIncremental);
}

using (var doc = PdfDocument.Load(@"e:\22\test.pdf", new PdfForms()))
{
	var pushbuttonField = doc.FormFill.InterForm.Fields[0];
	var pushbuttonCtrl = pushbuttonField.Controls[0];
	var widget = pushbuttonCtrl.GetWidget();
	var page = widget.Page;
    var icon = PdfBitmap.FromFile(@"e:\0\logo_square.png");

    widget.MK.NormalIcon.Clear();
	widget.MK.NormalIcon.Add(PdfImageObject.Create(doc, icon, 0, 0));
	widget.MK.GenerateAppearance(AppearanceStreamModes.Normal);

    //Save the document
    doc.Save(@"e:\22\test2.pdf", Patagames.Pdf.Enums.SaveFlags.NoIncremental);

}

SaveFlags.NoIncremental | SaveFlags.ObjectStream | SaveFlags.RemoveUnusedObjects

public static void TestSetButtonIcon()
{
    using (var doc = PdfDocument.Load(@"e:\22\test.pdf", new PdfForms()))
    {
        foreach (var pdfField in doc.FormFill.InterForm.Fields.OfType<PdfPushButtonField>())
        {
            if (pdfField.FullName == "image_context")
            {
                if (pdfField.Controls[0] is PdfPushButtonControl control)
                {
                    string imgFile = @"e:\19\d66ae37d46e00a1ecacfe9531986690a-9ba9622-2.jpg";
                    var widget = control.GetWidget();

                    //Create an empty image object
                    var pdfImageObject = PdfImageObject.Create(doc);
                    //Initiate it with a joeg file in order to reduce the image size
                    //(use packed jpeg instead of bitmap mask when the PdfBitmap is used)
                    pdfImageObject.LoadJpegFile(imgFile);
                    //Set the image Matrix
                    pdfImageObject.Matrix = new FS_MATRIX(pdfImageObject.Bitmap.Width, 0, 0, pdfImageObject.Bitmap.Height, 0, 0);
                    //Create/recreate the appearance stream
                    widget.MK.CreateEmptyAppearance(AppearanceStreamModes.Normal);
                    widget.MK.NormalIcon.Add(pdfImageObject);
                    widget.MK.GenerateAppearance(AppearanceStreamModes.Normal);

                    //The widget has its own appearance stream, which was generated based on the MK.
                    //Since we have changed the icon in MK, we also need to regenerate the appearance stream of this widget.
                    pdfField.Controls[0].RegenerateAppearance();
                }
            }
        }

        doc.Save(@"e:\22\test_output.pdf", SaveFlags.NoIncremental | SaveFlags.ObjectStream | SaveFlags.RemoveUnusedObjects);
    }
}