In this tutorial I will explain you in few simple steps how to create an encrypted image. So, here it goes:

1. Load the necessary kernel modules if they are not loaded already:

[root@florian home]# modprobe cryptoloop
[root@florian home]# modprobe aes

2. Create an image with DD. Size, path and file name is your choise. For size, you just modify the count value:

[root@florian home]# dd if=/dev/urandom of=storage.img bs=1M count=512
512+0 records in
512+0 records out
536870912 bytes (537 MB) copied, 68.6128 s, 7.8 MB/s
[root@florian home]# du -sh storage.img 
512M	storage.img

3. Encrypt the image with losetup and link it to a loop device:

[root@florian home]# losetup -e aes /dev/loop0 storage.img 
Password: 
[root@florian home]#

4. formate it (I was choosing ext4):

[root@florian home]# mkfs.ext4 /dev/loop0
mke2fs 1.41.12 (17-May-2010)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
32768 inodes, 131072 blocks
6553 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=134217728
4 block groups
32768 blocks per group, 32768 fragments per group
8192 inodes per group
Superblock backups stored on blocks: 
	32768, 98304

Writing inode tables: done                            
Creating journal (4096 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 37 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

5. Finally, mount your image:

[root@florian home]# mkdir /media/secret
[root@florian home]# mount -o loop,encryption=aes storage.img /media/secret
Password: 
[root@florian home]# df -h | grep secret
/home/storage.img     504M   17M  462M   4% /media/secret
[root@florian home]# mount | grep secret
/home/storage.img on /media/secret type ext4 (rw,loop=/dev/loop1,encryption=aes)

Enjoy!