I seen
this article and wanted to do it but I'm on vista.
On one note though this doesn't cover embedding an image because most email clients won't display them or blocks them so I kept my sig strictly html/css.
Also before starting this, be sure and change the signature from the default "Sent from my iPhone" to anything else so it will create the xml key we'll need later.
Anyways, after some looking around I found everything I needed online to do it without a mac.
First SSH into your phone to:
./private/var/root/Library/Preferences
and copy
com.apple.mobilemail.plist to your desktop.
But it's binary so we need to covert it to XML, so go to:
http://140.124.181.188/~khchung/cgi-bin/plutil.cgi
and it will covert it to XML and prompt you to save the converted file, you can just replace the current one on your desktop.
Next, open the
com.apple.mobilemail.plist in a text editor (I used wordpad).
Look for:
HTML Code:
<key>SignatureKey</key>
<string>*YOUR SIGNATURE HERE*</string>
This is where the css/html will be placed (obviously)
You can go here:
http://htmledit.squarefree.com/
to use a real-time html editor to view your work if you don't have a local html editor.
After you get something you like you need to encode it the xml won't take to the css and html tags outright.
So go here:
http://centricle.com/tools/html-entities/
and paste the css/html into the box and hit
Encode.
Now you can take that code and replace your signature text in the string.
So if you wanted to add:
HTML Code:
<div style="color: green">This is my sig</div>
It would look like:
HTML Code:
<key>SignatureKey</key>
<string><div style="color: green">This is my sig</div></string>
Save the
com.apple.mobilemail.plist and replace (backup original as it has other email settings in there as well) and your done.
Hope this helps.