The images need to be static resources in the salesforce instance, rather than part of the analytics template (who thought that was a good idea?!)
Next to your waveTemplates directory in force-app/main/default create a directory called staticresources and put your image file in there. Note if it's a JPG it prefers ".jpeg" as a suffix rather than ".jpg".
Also I think that it only allows lower-case names, so your example above wont work.
Create a matching metadata file in the same directory called <myfilename>.resource-meta.xml. E.g. if your image is abc.jpeg, create a file called abc.resource-meta.xml
Add the following content to that metadata file:
<?xml version="1.0" encoding="UTF-8"?>
Now you can add the templatePreview entry to the template-info.json file; the name value should be the file name without its suffix (E.g. "abc" from my example above). The image should not be declared in the imageFiles section of template-info.json.
You need to deploy to the Org from the force-app/main/default level so that it includes the staticresources directory.
Hope that helps!