As you enter the Facebook Designer, the Image Browser is launched. You can find an image based on its name, keyword(s) or type.
Browsing images by Type
All images are arranged by type. Use the category tree on the left-hand side of the Image Browser to browse images by type:
Cover Photos: These are larger images (815 x 315) that you can use as the Cover Photos at the top of your Facebook Timeline.
Tab Images: These are smaller images (111 x 74) that you can use as thumbnails for your page's tabs. These are shown under your Cover Photo.
Image categories can be navigated and expanded to view subcategories.
To expand a category and view its subcategories, click on the category.
To collapse a category and hide its subcategories, click the 
button to the right
of the category.
Once you've clicked on a category, all images in that category are displayed in the main viewing area on the right.
To view all images, click to the right of the first category.
All available images will be displayed in the main viewing area on the
right. The total number of images is displayed at the top left corner
of the image Browser.
Previewing an image
To preview a particular image, click the Preview button below the image. Once the preview opens, you can navigate through the previews by using the Previous and Next arrows.
Searching for an Image
All images are defined with several descriptive keywords. Images can also be searched by name. To search for a specific image, enter your search criteria in the search box and click the magnifying glass icon (or hit the Enter key). Matching images (if any) are shown in the main viewing area.
Selecting an image
Click the Select button to choose an image and open it in the image Editor.