@Martin,

Your HTML must include img tag with src attribute set to content-id of the attached image:

<img src="cid:logo@example.com" />

Also you need to add this image to Visuals collection:

MailBuilder builder = new MailBuilder();
builder.Html = @"<img src=""cid:logo@example.com"" />";
MimeData logo = builder.AddVisual(@"c:\image.jpg");
logo.ContentId = "logo@example.com";
IMail email = builder.Create();

Using fluent interface:

IMail email = Fluent.Mail
    .Html(@"<img src=""cid:logo@example.com"" />")
    .AddVisual(@"c:\image.jpg")
    .SetContentId("logo@example.com")             
    .Create();

You can find more details here:
https://www.limilabs.com/blog/send-html-email

@Martin

You can use Template.AddKey method:

string s = Template
    .Create("Show some number [key].")
    .AddKey("key", "42")
    .Render();

I’ve already implemented this feature, and it’ll be added in the next release.