How can I delete all these photos?

You apparently bought a used iPad since there is no reason why somebody else's photos would be on a brand new device. If the photos were synced to the iPad, the only way to remove them is by syncing again and you must either unselect the Sync Photos heading if you want to remove all of them, and then follow that with a sync. You can also select an empty folder on your computer in the Sync Photos from heading of iTunes and then follow that woth a sync to remove the photos.

if the photos were taken with the iPad or saved from emails or websites (camera roll photos) - tap on one of the photos, and the trash can appears, you can tap the trash can to delete the photo.

Another way to remove all the photos (and to generally make sure you have a fresh iPad for your OWN use) is to completely restore the device to factory settings. My suspicion though is that if the person you bought it from didn't know to remove their photos, they certainly didn't know about iCloud: Activation Lock. When you restore the device it will ask for the Apple ID and password used to originally activate it. Without that information, the iPad is little more than an expensive brick.

If the device belonged to the person you bought it from - and wasn't lost or stolen and then resold to you for example - this shouldn't be a problem. The original owner can follow the instructions in that link to remove the device from their iCloud profile. Once that's done, you can activate the iPad under your own Apple ID.