First of all, click Planner and then Posts. After you have been redirected go ahead and click Create new.

Choose your desired Pinterest account

Choose your desired photo Pin, then go ahead and click the little Pinterest logo on the top as shown in the picture and choose the desired board.

Once you are ready with your Pin go ahead and click Schedule for later.

Choose your desired date and click done

Click Schedule

Your Pin should be scheduled at the time you have chosen.