Muhammet, this seems not to be the bug of the theme. In a situation when the image is way too wide, safari behaves differently than firefox. If you would place a smaller image, you would see that both safari and firefox float it in exactly the same way.

If you really want to place such wide images in your posts, I suggest you use the available CSS classes described at the Sans-serif theme wiki page.

This is what I would use:

<div class="image wide"><img src="" ... /><p>Caption of your image</p></div>

Update: Please see the article above for a demonstration of an extra wide image inside a post. To have the same auto “cropping” effect you will have to update a single line in your style.php CSS file (Add overflow:hidden; to div.wide). See the theme’s changelog for details.