The plugin developer currently appears to have some real life issues to deal with, but I took the liberty of making a few small adjustments to make ERP use thumbnail images.
The relevant discussion on the developer’s site is here, and the code you need to make it work is here.
As always, you should verify that the code has only changed on the relevant lines (roughly: line 304-308 and line 410-413), and make sure that it only does what I say it does.
Also, note that future updates of the official plugin will almost certainly implement thumbnails in a different, more elaborate way, and that such updates might require you to handle thumbnails in your theme in a somewhat different manner.
And finally, I take no responsibility if my changes to the code have catastrophic consequences, like causing your server to grow sentient, form Skynet, and raise an army of robots to wage war on mankind.