Not very nice, because that's the icon for unknown palette items. Somehow you should be able to at least assign icons to the item, as well as, preferably, a display name and tooltip. I looked at the XML file that is generated by the IDE when adding a new item to the palette. That XML file does not contain an attribute for icons, hence the unknown icon is shown. I hardcoded the references to the icons directly into the XML file (using the info found in the NetBeans Code Snippet Module Tutorial), restarted the IDE, and now the items for my domain objects look as follows, which is much better:
I will look in Issuezilla and, if there is no issue for this, I will create one. Either the "unknown icon" should be nicer (simplest solution) or, ideally, the user should be able to select icons for their domain object palette items.
These were the two tags I added, reused from the "Choose Bean" item:
<icon16 urlvalue="nbres:/org/netbeans/modules/form/resources/palette/choose_bean_16.png" />
<icon32 urlvalue="nbres:/org/netbeans/modules/form/resources/palette/choose_bean_32.png" />