You cannot add a picture to a template within the current Template Editor, that is if you expect a WYSIWYG-kind of editor. However, there are two workaraounds that may help you:
When writing a message (in the Message Editor window), you can save the message for later editing (the "Save"-button). When saving a message, you can choose to save that message as "Reusable mail stationery".
The workaround is simply: just write a message within the Message Editor window (as if you wanted to send it); add pictures, formatting etc. as you want to; save the message, checking "Save this draft as reusable mail staionery". Next time you want to send such a message, open the Draft Manager window, edit the message text and the recipient's address, and send it. The Draft Manager lists those messages as "Stationery" (unlike a "Draft message" that has been saved only for later editing).
Note: if you open a message from the Draft Manager, make sure that the "Rich box"-box is checked. There is a small problem in saving a formatted message: the "Rich text"-box is not checked anymore if you open the message from the Draft Manager (even if you checkmarked that box before saving the message), so Pegasus Mail does not know that you want to send that message with formatting and inline-graphics.
If you want to have a fancy signature added to the message, make sure you have a "Rich text"-variant of the respective signature set defined.
I have to admit that the following workaround is something that I have not tested much, so you may have to try and find out whether it helps you. You should also know that the following suggestion is rather technical, so you may want to follow it only if you are (more or less) experienced in HTML and in the formal syntax of an e-mail mesage (like message body, message headers, inline-graphics etc.)
The basic idea is to insert the HTML source code to the Template Editor and to tell Pegasus Mail to send an HTML-message. The main problem here is that the Template Editor currently does not have an option to switch between HTML and "text only", neither is the Template Editor a WYSIWYG-kind of editor.
(b1) The first thing is to enter the MHTML source code. MHTML means a mail-targeted version of HTML, and in this regard, it also means the additional information like boundary lines within a multipart message, content-type information etc.
Send yourself an e-mail message with inline graphics, fancy formatting, etc., and look at the raw view of that message after receiving it. You will probably see some lines similar to "------=_NextPart_000_0040_01C810B1.918D9510" or "Content-Type: image/jpeg;". All of these lines and sections follow the message headers. The idea is to have the same structure within the "Body"- / "EndBody"-command of the Template Editor (including the decoded version of the picture file you want to send within the message). Perhaps, you send yourself the very message that you want to send as a birthday message and simply copy its raw vew to the "Body"- / "EndBody"-section of the Template Editor after receiving it.
Note that the Template Editor now contains the MHTML-source code which is unusual since the Template Editor usually "pure text" only. In other words: what you are doing here is a kind of a hack.
(b2) Usually, the Template Editor expects a "text only"-message to be defined. However, the message you want to send is an HTML-message, so you have to tell Pegasus Mail (and the recipient's e-mail client) that the message is an HTML-message instead. If you did not do so, Pegasus Mail would simply send the MHTML-source code as it is, so the recipient would simply receive the source code, not the fancy version you actually want to send.
In order to tell Pegasus Mail and the recipient's e.mail client that the message is HTML, you add the line similar to
set Header "Content-Type: text/html; charset=iso-8859-1"
to the template. Look at the sample message you have sent yourself: what "Content-type:"-header (within the actual message headers) was added to it? You may just copy that line to the template.
- This is a really technical hack. A downside of it is that the message the template will generate will probably have two "Content-Type:"-headers. I am not sure whether all e-mail client can handle this.
- While creating the message based on that template, Pegasus Mail handles that message as a "text only"-message. Because of the hack suggested here, it is only the recipient (not you) to see the fancy version of the message. I also suggest not to add any signature in a hack-template like this one.
As (b) IS rather complicated, I suggest you might send me a copy of the picture you want to add to the template, so I can try and create a template based on the hack I have suggested. If I succed in creating such a template, I will make it available to you (of course); otherwise, I will tell you that it does not work. As you said, you are new to Pegasus Mail, so I think a hack like this one may be too demainding to start with for you.
If you want me to create that template for you, just send me a private message, so we can discuss details.