Image size DW Focus in slider and blog post

Hi,
If I post an image on the homepage of DW Focus I have the following problems:

• The photo seems to have a size of 400×600 when I measure with a screenshot
• When I look in the CSS it says 400×710.
• So always a part of the picture is taken off on the side…
• Why is this? And what can I do about it? Do I need to change the code for the slider?
• And I also want to be sure that the photo in the blog post (which is shown bigger) still looks good.
• So what’s the recommended image size for slider and blog post?

Thanks!