[djbaxter]

Update:

It appears that the error messages I received and posted above were misleading (at least they misled me). It is not the case that this add-on is finding the same missing image in a number of posts; I thought that initially made sense because the image pointed to a no longer available smiley.

Rather it seems that once the add-on encounters a no longer available image, it gets "stuck" on that image each time it tries to process another run, and keeps generating the same error.

A work-around which appears to be working for me is to add the URL (or part of the URL) pointing to the problem image to the add-on's Ignore list:

AdminCP >> Settings >> Options >> vB4 Import External Images >> Ignore

Ignore
Images containing these values are ignored and will not be replaced. Normally you will enter here your media server or gallery folder. Images of your forum and the imported folder are always ignored. Please separate each item by a new line.

My ignore list now looks like this:

imgcache/
websmileys.com/
/shea0017/
allposters.com/

Now that I'm posting this, I'm not sure what "imgcache/" is - that was prepopulated by the add-on.