Thank you for your help.
I'm facing the issue in Nintex Workflow 2013.
I performed product update and after that, "Workflow History" button's image is not displayed.
(See the below image. My farm is not English environment.)
I checked there was the image file in hive.
So I'm not sure why the icon is not displayed.
Need your help on this issue.
There is currently an open defect for this issue.
The problem stems with trying to find the required files in the 14 hive and not the 15 hive. You can deploy the Nintex Workflow Backwards compatibility solution to resolve this in the meantime.
Let me know if this helps.