imageres.dll is an icon library including icons for files & folders. It is a Windows Image Resource, located in the C:\Windows\System32 folder. 

If you ever wanted to add a new set of customized icons to the imageres.dll file in Windows, watch the below video from my YouTube channel. You need to download the Resource Hacker software http://angusj.com/resourcehacker/#download  & you can convert your image with ICO Convert: Convert your image to ICON