Using an image as signature is not possible - instead it throws

Timestamp: 8/2/2013 6:05:04 AM
Warning: readSignatureFile - exception trying to read signature attachment
file; expected charSet = UTF-8 !
Either save your signature with this charset or can change it through the
config setting extensions.smartTemplate4.signature.encoding
C:\Users\XXXXXX\Pictures\Business_Card.png
Component returned failure code: 0x8050000e (NS_ERROR_ILLEGAL_INPUT)
[nsIConverterInputStream.readString]
Source File:
Line: 47

I can reproduce it, expect a bugfix soon.

Created an attachment (id=7638) [details]
0.9.4 Prerelease 8

This version checks if you attach an image file by doing a simple name check:

if file name contains one of .gif .tif .bmp .jpg .jpeg or .png

it will insert the image using an <img> tag.