If you make a full backup on the external drive, then your iPhoto Library will be backed up. However, I discourage you from using a backup drive as a simple storage drive. You should use separate drives for each.
I you don't plan to make the change, then you need to create a partition on the external drive large enough to hold the backup of your internal drive.
Drive Partition and Format
1. Open Disk Utility in the Utilities folder.
2. After Disk Utility loads select the external hard drive (this is the out-dented entry with the mfgr.'s ID and size) from the left side list. Click on the Partition tab in the Disk Utility main window.
3. Under the Volume Scheme heading set the number of partitions from the drop down menu to Two. Click on the Options button, set the partition scheme to GUID then click on the OK button. Set the format type to Mac OS Extended (Journaled.) Resize and name each partition. Click on the Apply button and wait until the process has completed.
Clone using Restore Option of Disk Utility
1. Open Disk Utility in the Utilities folder.
2. Select the destination volume from the left side list.
3. Click on the Restore tab in the DU main window.
4. Select the destination volume from the left side list and drag
it to the Destination entry field.
5. Select the source volume from the left side list and drag it to
the Source entry field.
6. Double-check you got it right, then click on the Restore button.
Destination means the external backup partition or volume. Source means the internal startup drive.
This should make a bootable clone, but when you are finished you should try booting from your clone to be sure it works.
Boot Using OPTION key:
1. Restart the computer.
2. Immediately after the chime press and hold down the
"OPTION" key.
3. Release the key when the boot manager appears.
4. Select the bootable clone you just made.
5. Click on the arrow button below the icon.
If all is well, then open Startup Disk preferences, set your internal drive as the startup drive, then click on the Restart button.