From ModMyi - Wiki

Step 1

Download iThmbConv.exe from here.

There are also a couple files that you need to download off of your iPhone before you can convert them. Use a program like iPhoneBrowser or iBrickr.

The files are:
Photo Database
F3004_1.ithmb
F3008_1.ithmb
F3009_1.ithmb
F3011_1.ithmb

They are located at:
/var/root/Media/Photos/
and
/var/root/Media/Photos/Thumbs

Step 2

Copy all five files and the program to:
C:\tmp
(Note: the directory is not important, but this is for simplicity.)

Step 3

Click Start > Run.
Type "cmd" and hit enter.
This will bring up the command prompt.

Step 4

Now you need to navigate to the folder that you downloaded the five files at.
Type "cd c:\tmp" and hit enter.

Step 5

Now type "iThmbConv.exe"
There will be a list of examples on how to use the program.

Step 6

Type "iThmbConv c:\tmp\F3008_1.ithmb" and hit enter.
Note: be sure that this file is the largest one you downloaded, if it isn't, choose the one that is.

Step 7

You will now have a bunch of files in your tmp directory that have a .ppm extension.
You can open these files using Photoshop and save them as you want.
You could even use a batch to convert them all at one time.

XnView can view .ppm files, as well as batch convert them to other types.

Step 8

Alternately, simply change the file extension from .iThmb to .jpg, then your good to go!