Varies between phones, but you can go to the Gallery, locate the picture you want to delete and then on some phones, tap Menu (three vertical dots) and select Delete on others, long press the picture then tap the delete (bin) icon.
Alternatively, if you need to delete a lot of pictures at once, you could use a file manager from the Play Store (I use a free one called File Manager) or you could connect your phone to a PC via a USB cable and use Windows Explorer (Windows key + e).
