A. Gallery provide a shortcode [a_gallery]. With attributes you can easily customize size of thumbnails and gallery slider.
[a_gallery] and upload some pictures (do not insert it in post
if you don't want add images in post content).
If post have attached photos you can detach it by clicking Delete link located below of item in "Manage images" meta box.
There are two options: 1. Upload pictures from edit post page and save changes, but do not insert it in post if you don't want add images in post content. 2. Upload pictures from media tab in admin and then navigate to post and in "Manage images" meta box click "Add from library button". Then you can select pictures and save it.
You can customize:
[a_gallery post_id="1" count="7" border="3" border_color="#ff0000" item_h="100" item_w="120" columns="3" exclude="2,3"]
will insert a gallery with 7 images attached to post "Hello World!" (post ID = 1), exclude attachments with ID 2,3. Slider will
contain 3 visible and 4 hidden thumbnails with size 120px by 100px and
#ff0000 border of 3px width.