Delete a column in a list missing a delete button

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.