To display images in your pages or posts, first add them as attachments to the corresponding card.
Then get a direct link to them. This can be done in various ways, but a quick one is to right-click at the image in your card's attachment link and select "Copy link address" or something like it. Another way is to perform a middle-click (or Ctrl+click if you don't have a mouse) at the attachment link and copy the URL of the page that opened in another tab at your browser, which will contain the raw image, directly, just the image.
The URL will look like this:
Finally, in the card description (where you write the page/post content), write the following magic:
![image description](http://the-image-url-you-copied-in-the-previous-step/) ("image description" can be blank, so it will be
That's it. You will notice that even in the Trello interface, the attached image will be displayed in the middle of the card's description. That's the sign of success.
After adding an image as attachment to the Trello card, remember to click at Remove Cover, unless you really want that image to be the cover of your post/page in your site.