What is the recommended size/pixels for pictures on the side panel?
We have recommended image sizes listed in our FAQ as questions for each story map app like:
What are the recommended image sizes for a Map Journal?
The main thing to keep in mind is that resizing can help, but compressing your images usually gives you the most performance gain.
Also note that these recommendations only apply when you are hosting images on your own web server. If you use photos from Flickr or upload images directly in the story map builder those images are optimized for you, so you don't need to do anything to them, regardless of their original sizes.
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