I found a way to add a signature image to a form without doing any codeing, but it isn't eloquent.... My goal for this exercise was to have a SharePoint list that contained a digitized (image) signature for users and managers to be inserted on a form; which I've accomplished.
Infopath Limitation: When using a picture box control, the control will only accept encoded binary data. Using a RichText control allows a URL to be passed, but it encodes the "<" and ">" so it won't translate correctly with the encoded replacemenets "<: / >" in the XML. (see above comments)
Solution: Store the encoded binary data for the picture in the SharePoint library. In SharePoint I created a Picture library and added a few extra columns (User and PicData). The "User" field matches a user's AD user name string so I could do an easy look up to match the current user to their signature. PicData is a multi-line text field with "no size limit" selected. By placing the encoded binary data for the image in this field, I am now able to define a DataConnection back to my Signature Library and retrieve the encoded data. I tie a rule to a checkbox to update the 'appvlSignature' control on my form. This way, when the approver checks the "isApproved" box the binary image data is stuffed into the picture control, thus updating the image.
It's not a great solution, so no flames.... In my case I have around (50) signatures, so the set up to encode the files it's too cumbersome. Additionally, by using this approach I don't need to utilize a developer, nor do I need to manage any extra code in the form. This means I can train ordinary office staff (familiar with Infopath) to use the built in functions without the need for special coding; It's just a matter of creating the data connection, and writing some basic rules.
andy t.