Dear ptbh,
Your question breaks into two. The first is about referring to a picture on any Web page, no matter whether a content manager (e.g. MODx) is involved in composing the page for you. The second is how to use MODx to make that convenient to do whenever you desire, and without "breaking" your site.
The answer to the first question is to study use of the
tag.
HERE is a nice place to start. It explains the tag and offers some examples you can try. A Google search will uncover many such citations.
The answer to the second is to study how you can guide MODx to dynamically assemble pages for you. In particular, your Web site designer might build a
template variable for an image into the
template for your blog entries. Or, if you want to include an image just once, your designer might show you how to use the TinyMCE editor’s "Insert / edit image" button to create and position a reference to your image, using that "img" tag, within a specific blog entry.
......
The fun of this Web stuff is that you can try out anything imaginable for free at zero risk, and you can skim the surface or dig as deeply into the technology as you want. You can begin by typing some tags into a text file, then asking your browser to interpret them before your very eyes. You can progress to applying a programming language (e.g.
PHP) to creation of those tags. You can further use the programming language to query and/or update a database (e.g. one based on
MySQL). You can even simulate a Web site hosting service on your own desktop, to privately practice your Web work (using perhaps
WampServer or
XAMPP).
Having created Web sites for a while, you will find yourself doing certain things over and over, such as creating menus of links to facilitate clicking from one page to another, or formatting a bunch of items (blog entries, maybe?) exactly the same way, again and again. (Tedious!) Then you will look for help in mechanizing the routine stuff, so that you can concentrate on the valuable content instead of fiddling with tags and programming. That is where MODx and related content managers come into the picture. All content managers are built upon the basics described in the previous paragraph, so it is helpful to have some feeling for those basics as you use MODx.
Have fun with it!