You can delete photos from the Saved Photos album, which contains photos you save from email or the web. Tap the square symbol with the arrow at the top right of the screen displaying the thumbnails. Tap the photos you want to delete. A white checkmark in a blue circle will appear on the selected photos. Then, tap the red delete button.
For photos synced from your computer, you need to delete the photo from the album on your computer, then sync iPad again. See the section titled "To delete photos from your device" in this article: iPhone, iPad and iPod touch: Syncing photos via iTunes, http://support.apple.com/kb/HT4236