1. Did you use relative URLs within your story's passage to reference the images?
eg. If your images are stored within a media sub-folder then the URL of your img element should look like the following.
2. File and Folder names on that website (and most others) are case-sensitive, did you make sure not to use mix-case letters nor space or special characters in their names? It is OK to use a standard dash or an underscore in place of space characters.
3. Did you use the ZIP File method mention on the Uploading HTML5 games web-page to upload your Twine based story to their site?
4. Did you include the images used by your story within the ZIP file, and are those images within the correct sub-folder within that ZIP file?
note: Obviously both the Folder(s) containing your images and the Images contain within them may have names different than my examples.