Can anybody tell me how to add a preview image when i am selecting a rendering while adding a component in the Page Editor? I just still have there a default sublayout icon. I have the Nicam demo site from Sitecore and there are some images, but I am not able find where I can select that.
When you go to your Sublayouts (/sitecore/Layouts/Sublayouts or Renderings) you can select yours, go to the Appearance section and go to the Thumbnail field. Click the Take Screenshot button there.
You can then select the page you want the thumbnail to be taken from in the Item field, which device to use and click Take Screenshot. After this the page will show up where you can select a section of the page to be used as the screenshot
[Update] I decided to write a blogpost on this: Read it here