When uploading a new image, you have the option to choose wether a comment should be immediately published, sent to the moderation queue, or disabled all together.
These settings will only take effect for the current image you are uploading and will override the Global Comment Setting if it's different.
Alright, so you set one of your images to "Publish Instantly" but now you want to "Moderate" the comments for that image. Changing the comment setting is easy!
Find the image you wish to change within the Images menu and click the [Edit] link. Within this page you will be able to change the previously set "Comment Setting".
The "Global Comment Setting" determines the default setting for all future image uploads.
If your Global Comment Setting is set to "Publish Instantly", than all new images uploaded will use this same setting unless changed otherwise.
It is important to note that the Global Comment Setting does not effect images uploaded using versions of Pixelpost prior to v1.6.0
To change a images comment setting that was uploaded using Pixelpost v1.5.x or earlier, you can use the instructions above on Changing the Comment Setting or you can download the Global Comment Settings addon to perform a mass update.