I ran into this for the first time today. All of my images from the Media Library were rendering fine in every browser except Safari on the Mac. The URL for the images was being rendered using the default ASHX extension. To fix this, I went to my web.config and removed the value in the Media.RequestExtension setting. This will instruct Sitecore to render the Media item URL using the extension of the media item.
Now, all my images are displaying correctly in all browsers.
<!-- MEDIA - REQUEST EXTENSION The extension to use in media request URLs. If the value is not set, the Extension field of the individual media items will be used (ie. JPG, GIF, etc.) Default value: "ashx" --> <setting name="Media.RequestExtension" value=""/>