More information about your setup would be a great help!
Things to check:
* Permissions: images need to be able to be read by the webserver. Check to make sure they are readable on the file system. If you're using FTP try setting the owner as the webserver user (www-data or apache) and permissions should be 644. The easy way is to ensure permissions on the whole assets dir is 775
* DId you install in a subdirectory? If so then it could be a path issue. Check the path of the image in the final html and try and access that in a webbrowser.