Excel for Mac 2011: Add a Picture to a Worksheet Background
Behind the cells on each Excel worksheet in Office 2011 for Mac is a background layer that you can format. Excel for Mac 2011 doesn't offer a lot of customization to this feature, but you can place an image on the background layer by following these steps:
Choose Format→Sheet→Background from the menu bar, or go to the Ribbon’s Layout tab, find the Page Setup group, and click Background. (This button may not appear on the Ribbon if your workbook window is small in size.)
The Choose a Picture dialog appears.
Click the Browse button and navigate to a picture you want to use as a tiled background.
While you can use any image, keep in mind that Excel will tile the image, which may not be the desired effect. Also, images that are busy or too dark will make it hard to read any data in your worksheet. You may want to stick to light-colored, simple images.
Click OK to place the image.
If you decide you don’t like the image in the background, you can remove it by going to the Ribbon’s Layout tab, and in the Page Setup group, click the Background button. Remember that this Background button is a toggle.