I ran into problems when adding resources to a plugin. I had to restart Lr when adding any resources, at least on a Mac. It appears to be a known issue of some kind.
I'd also check file permissions and double-check the filenames being used, both on the filesystem in the plugin and in your publish definition.
I had the same problem: branding image file small_yag.png not found. Restarting lightroom did the job, now everything works fine.
Some keywords: publishServiceProvider.small_icon = 'small_yag.png'
I've never *noticed* a problem with icon resources, not that I would have, but I have noticed the problem with other plugin files, on Mac. Its almost as if Lightroom takes a snapshot of what's there then ignores new stuff when you reload the plugin.
e.g. ExifMeta re-writes the metadata definition module. On Windows, simply reloading the plugin incorporates the newly re-written module, whereas on Mac, Lightroom must be restarted for it to be incorporated. Perhaps these issues are related.