SteveB

Still perplexed. I found that the admin's edit page embeds this javascript. Seems pretty indiscriminate to do that to all the img tags.
$(document).ready(function(){ $( "img" ).attr({ src: "", title: "Processwire.com", alt: "Processwire.com", height: "23px", width: "auto" });}); Note that it clobbers the src attribbute.
Haven't yet found where that comes from.
FIXED!
It was the AdminThemeCustomizer module (version 0.0.1). Uninstalled that and no more problem.
Weirdly, I had noticed that months ago and forgot about it.

Ryan: All good with the latest dev version.
I had started this with the "stable" code and the Blog Profile. Then I realized I needed a feature from the dev version and I had a copy of dev that was about month old sitting on my hard drive. I dragged the wire folder over to my project. That gave me the feature I needed and mostly wortked but I had the non-object error. I downloaded a new copy of the dev version on 6/21, dragged the wire folder in and that fixed it.
Soma: Re. "normal login practice"
Having come to PW fairly recently I look for examples to see what the common idioms are. If I see similarity in how people go about a process, especially such a universal one as logging in, that tells me something. All I was saying is that I hadn't seen much on the topic until I found Ryan's post.
P.S. I'm impressed with the high signal to noise ratio of this forum and PW itself.