1. Juxtapose uses iframes to add dynamic images, so you will first need to create a simple page in Omeka (using the Simple Pages plugin) to house the iframe image.
2. You will also need to allow Omeka to use iframe elements: in security settings, add "iframe" to allowed HTML elements, and uncheck the HTML filtering box.
3. Juxtapose uses images that you upload from Dropbox to the Juxtapose site, or that you link to from the web. I found the latter to be the easier option.
To link to an image from your Omeka site, you will need to find the image's url:
4. In Juxtapose, add labels and credits, and preview the image. When it's ready, click "Publish." This provides a web link and an embed code. Copy the entire embed code.
5. Back in Omeka, go to the edit page for the simple page you created. Click on "HTML editor" in the box on the bottom, and paste the iframe embed code you copied from the Juxtapose page. (I also added a <p> element around it. Here's why: I did the other steps first, but the only thing that showed up on the public page was an empty box. When I viewed the html editor, I saw the iframe within a <p> element, with a basic empty box structure. So I tried adding the <p> tags, and it displayed properly on the public page.)
6. You will need to adjust the iframe dimensions to make sure it displays on the whole page and adjusts size.
In Omeka support forum, I found this post helpful:: https://forum.omeka.org/t/adding-iframe-to-simple-pages/855
If you want to add a simple and dynamic timeline to your site, try using TimelineJS. (An example can be found on Explore Fisher.) Timeline works by using a Google sheet to input image links and data and then converting them into an iframe image. A template sheet is provided for you, or you can create your own so you can customize the timeline further.
Tips for using Timeline in Omeka: