Preview image of developed theme don't show

Dear Plone Masters,

I'm studing Plone and started to develop a theme using the mrbob. Everything ok, but the preview (preview.png) image don't show in @@theming-controlpanel, just a white retangle with a X.

Someone could help with my mistake?

Thank you.

Can you paste your theme manifest.cfg?
What is the link of the broken img you read in the html source of the @@theming-controlpanel page?

Hello, Mr. Alessandro Pisa,

You did the trick! The mrbob tool miss the line "preview = preview.png" inside of manifest.cfg file. I'm put it back and the preview works now!

Thank you!