Hi Lindsey,
You can save the PictureBox.Image to a stream and then insert that stream into a worksheet.
For example:
var stream = new MemoryStream();
this.pictureBox1.Image.Save(stream, System.Drawing.Imaging.ImageFormat.Png);
stream.Position = 0;
worksheet.Pictures.Add(stream, ExcelPictureFormat.Png,
new AnchorCell(worksheet.Cells["B3"], true),
new AnchorCell(worksheet.Cells["E6"], false));
I hope this helps.
Regards,
Mario