I'm gonna explain my way to do this, will not guarantee this is the fastest or whatever best way.
1. Write them articles, assign them to a section/categorie that you will not need otherwise and name it like "hidden articles" for better overview. (or leave them uncategorized)
2. I'm using JCE
http://www.joomlacontenteditor.net/
not the joomla default editor! Download URL
http://www.joomlacontenteditor.net/downloads/editor/joomla15x/183-comjce157154package
You have to enable it in the > globals >site right after installation.
3. When editing an article in JCE and having an image placed in this article, click on this image so that it is marked, click the "insert link" button, the "advanced link" window will pop up, select your article and it's done.
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If you don't want to deal with JCE, then temporarily asign the category to a menu; watch the articles in your browser, you can see the url now in your browser. Write it down or copy it. Later just unpublish the menu item.