For sealed columns (e.g. Rollup Image, etc.), there's no way to delete them from the UI. But you can easily get around this if you use the following powershell command:
$web = Get-SPWeb -identity http://site/name/
$list = $web.Lists["ListName"]
$column = $list.Fields["Rollup Image"]
$column.Sealed = $false
Then that Delete button will be visible and you can go ahead and delete that column.