Suzanne
The Help file says you cannot insert a graphic into a header.
You can, however, use some of the picture type WingDing symbols at a large font size if they are suitable.
A substitute would be to insert a picture on a row (or rows) at the top of your spreadsheet and use these as a title. For this you need to go to Page Setup, select the sheet tab and ensure your print area does not include these title rows. Then in the box labelled "Rows to repeat at top" put the row/s that contain your graphic. This will then print out at the top of each sheet.
Hope this helps
Derek