problems with background images

I've set up an interactive spreadsheet for kids where labels need to be
typed in cells over a background image. I have the column widths and row
heights adjusted to align with the image. However, when I close the
workbook and reopen it, the image appears in a slightly different
position. I can get the image back to its original position by deleting a
cell - any cell on the sheet - and when I undo that delete, the image is
still in the right place.
Can anyone explain this strange and annoying behaviour?

Marilyn