You have two options to add a single picture in a page or blog post:
NextGEN Gallery Single (this allows you to add a single picture from any gallery you have created previously) -
1) Click on 'Add Media' in your text edit toolbar on your page/post
2) A pop-up window will appear, click on the 'NextGEN Gallery' tab. Here you may select from any gallery and all images will display below in thumbnail list for you to select to insert into your page.
WordPress Media -
1) Click Add Media
2) Drag and drop or browse from your computer to upload image to your site
3) Select your image to insert to your page or post
4) Click Publish/Update when you're finished!