You can delete them using the gallery, however you have to do this one at a time.
The easiest way, though is to install a file manager from Play (I use one called File Manager that is free and has the advantage of displaying thumbnails of image files). On stock Android, photos are stored in the DCIM/camera folder. If your images aren't there, you can use the search function within the file manager to find them.
Note that, depending on which phone you have and how you have it set up, your pictures may be stored on the SD card.
