a highly customizable WordPress-plugin that shows a random image anywhere on your blog.
Have a look at the enclosed html-templates in the template-folder. For advanced syntax (e.g. if-else) there's the mustache(5)-manual on the Mustache-Site
This might be because your wordpress-theme's css has an influence on the output of the plugin. You might have to adjust the css-template (set margins to zero, stuff like this)
Just add a text-widget to the sidebar and insert the shortcode there
Check if you have a cache-plugin running. If the page is cached, there won't be any random images on reload!