If you want to include images in your messages and formatted text then you will need to create your message using HTML (Hyper text mark up language - used to construct web pages).
If you are using Group Mail Pro or Plus then you can use the free GroupHTML plug-in to help you create such messages. Alternatively you can use any HTML Editor. Just make sure that the HTML message is saved as a file. You then just need to select that file in Group Mail as the optional HTML message file (see the bottom part of the Edit Message tab).
Group Mail will take care of the rest.
Please note that if you do not include a text part also one will be generated automatically by Group Mail.
Signatures are not applied / embedded in HTML messages - they are only used with text messages.
All local images will be wrapped up with the message (aka inline images). Some web based email clients are not able to properly decode these. If this is a problem for you then you should consider placing the images on a web server and then link them into your HTML. This will make your messages a lot smaller (faster to send) and each recipient will download them as required (this method could be used to check on viewing stats - by checking your log files you will see how many, but not who viewed your message in HTML).
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