First
of all, find the image you'd like in the gallery and
click on it.
You
will be taken to a page for this image.

At
the bottom right of the page, you will
see a link "Add
to shopping cart".
Click it if you would like to purchase the image.
The
Shopping cart
You
will now see the Shopping
cart
page, which will look like this:

Here
you can manage your shopping basket or return to the
gallery.
If you are done shopping and wish to pay, click "Continue
to checkout",
at the bottom of the page.
Checkout
and payment
You
will now see the Checkout
page, where you can enter your payment details.

Select
PayPal from the list on the right
and read and agree to the terms and conditions.
Then, enter your payment details in the blue
area.
When
you are done, click the "Send
order and pay"
button at the bottom of the page.