The easiest way is to open the Facebook app, tap Menu>Photos, select the album, and tap Add Photos.
If you're trying to share the photo directly from your Gallery or Photos app to your Facebook album, then select the photo, tap the Share button, and select Facebook. Tap the "To" field, and select Only Me--no one else will see it. Then, back in the sharing screen, tap the icon at the bottom row, second from the left. That's the Album icon--you can select which album to post the photo to.