I'm not sure on the timing of the copying of resources to the target dir, but it seems that the images simply weren't there in time for the copy. I realized that there was no real reason to need to copy from the target directory instead of my source. I updated the directory as shown below and the images were successfully copied over.
Before:
<directory>target/${project.artifactId}-${project.version}/plugin/resources/${plugin.client.version}</directory>
After:
<directory>src/main/webapp/plugin/resources/${plugin.client.version}</directory>