1 – find image id
To get your image ID# go to your Media Library. Then find the image you want. Just roll over it. You will see in your status bar (at the bottom of your browser) the ID# appears. Make sure you have Show Status Bar enabled on your browser. (if that’s an option for you).
2 – add gallery exclude=’xx’ to gallery html
(the gallery html is really a point to a style sheet – i just added the gallery exclude clause:
)

How to Exclude Specific Images:
If you want to include all images attached to a post, with the exception of a select few, you can use “exclude=” in the shortcode, followed by comma separated attachment IDs:

How to Include Specific Images:
If you want to include only a few specific images, then you will want to use the “include=” option in the gallery shortcode, followed by comma separated attachment IDs: