The bootswatch themes meet all of my needs except for one-- using a graphic logo at the top left hand of the navbar, albeit a small one.  I guess maybe the reason it isn't done is the mobile first nature of the theme, which I understand, but its a really tiny PNG of an olive branch...

So if I have a bootswatch theme set up as I want otherwise, where should I "hack" around to get a PNG up there?

Try something like this in your CSS if you want to keep the text and add an image next to it:

a.navbar-brand {
  display: block;
  background: url(images/olive-branch.png) no-repeat;     //link to image relative to style sheet
  padding-left: 100px;      //width of image
}

Try something like this in your CSS if you want to hide the text and replace it with an image:

a.navbar-brand {
  display: block;
  width: 150px;     //width for your image
  height: 0px; 
  overflow: hidden;      // text is hidden
  background: url(images/olive-branch.png) no-repeat;     //link to image relative to style sheet
  padding: 50px 0 0 0;      //top margin is height of image
}