Warning in Admin Dashboard and an enhancement request

Hi, I am getting below warning on my Dashboard page. Though the The plugin is working fine

Warning: call_user_func_array() expects parameter 1 to be a valid callback, function ‘steem_plugin_backend_js’ not found or invalid function.

It’s a good work been done by you. I was also trying to create a similar functionality.
I have a enhancement request: Show thumbnail images of the posts shown in the feed
It would create a better user experience!