How do I delete photos on my iPad?

Yes, I've googled, but still can't work it out

A really long time ago I set up some kind of sync via iPhoto, can't remember exactly how it was. I no longer use iPhoto and all the photos I had in iPhoto on my mac have been transferred to folder that I organise with Lightroom.

I want to get rid of all the photos on my iPad, because I never use them and they just take up space. The delete-icon in the Photos app is greyed out. I've never synced via iTunes, so unchecking in there isn't an option.

What else can I do?