I don't think there are many user friendly ways to upload and serve arbitrary images for linking/embedding in a Document Page entry. Here are two methods that have worked for me:

WYSIWYG editor in raw HTML mode

Based on examples from the OpenERP demo data in the Document Pages module, you will need to edit the HTML code of your document page in the editor to type img tags rather than using the WYSIWYG editor features alone.

External image service

You can specify an external URL to a photo sharing site, local HTTP file server, or Amazon S3 Bucket, etc., as in <img src="http://example.com/image.png">

Base 64 encoded data URLs

Paste a lengthy <img src="data:image/png;base64,iVBORw0KGgoAAAA…PAAAAABJRU5ErkJggg=="> tag generated by base-64 encoding your image. CSS rules can also add background images from base 64 data. There are external online tools to generate the img tags and CSS rules.

Make your own quick OpenERP add-on

If you really get sick of embedded data URLs and external hosting, you can drop a folder of contents onto the server bundled as an "add-on" and have everything in the "/<addon name="">/static" path be served up. It's very easy and requires no programming knowledge, but does require write access to the file system and the ability to tell OpenERP where your extra add-on is located (that aspect is document well elsewhere).

Add-on folder structure:

my_files/
├── __init__.py
├── __openerp__.py
└── static
    └── example.png

__init__.py is empty:

__openerp__.py contains:

{
    "name": "My Static Files",
    "version": "1.0",
    "description": "Static file collection bundled as an add-on.",
    "author": "Arnis",
    "depends": ['web'],
    "installable": True,
    "auto_install": False
}

1. • If you want to point OpenERP at an additional addons folder, edit your server config file (and restart) or change the command-line switch, --addons-path to point at the extra add-ons folder where your add-on's folder is contained
   • If you dropped your add-on folder into an existing add-ons folder, click Settings / Update Modules List
2. Locate and install your add-on from the Settings menu
   UPDATE: It seems installation is not necessary, at least in the trunk I tested with today (8.0).
3. Your add-on and the URLs should activate. You should be able to view an image in the browser with a URL like http(s)://openerp.example.com/my_files/static/example.png and img tags should likewise work as expected.