I believe you can turn that option on/off in the style properties under the node settings. That's assuming you are trying to get the ultimate custom nodes plugin to function. I could be wrong, but I am sure I ran in to this issue as well.
I don't have any problem. All icons are working well, may be because of I have not used any third party plugin to change the node icons. Take a look here: My forum . In fact I am using the default node icons of the theme itself.