Read Theme Unit Test and import the test data (including images!) into a new blog. You will get everything you need to test.
There is a gallery post included (Images Test), and if you click on an image you get an attachment page for this gallery image.
Here is a screen shot from TwentyEleven:
See the links in the upper right corner? Look into the source code of TwentyEleven to understand how they are implemented.
Use previous_image_link()
and next_image_link()
.
<span class="nav-previous"><?php
previous_image_link( false, __( '← Previous' , 'twentyeleven' ) );
?></span>
<span class="nav-next"><?php
next_image_link( false, __( 'Next →' , 'twentyeleven' ) );
?></span>