1.) How can I restrict access to this blog. I want, that if somebody opens the page, and they have to register first. Is it possible? If found no option to set this up in the backend. If its not possible, can rectrict at least commenting by registering first?

You can restrict comments. You have to do a search on how to do that in the forum or play around in the back office.

2.) I really like the nifty_corners skin, but it doesn't work because its for a newer version. Can I downgrade this skin? I only found upgrade descriptions.
And the link on the skins page to 0.9.x skins is broken.

You should upgrade to 1.10 at least - using .0.9.x is a bad idea since that's a really old version. All the cool stuff will be on v2.

3.) Can is there a way to let b2evolution automatically place the pictures so I don't have to use resizing and aligning?
This is a CSS styling issue. having a "one size" for all may be possible via CSS (skin template) but probably a bad idea.

You can insert a picture and use the width="80%" and that will size the picture on demand for you.

4.) I tried "justify"-alignment for the text but it didn't work. Did I something wrong, or is it not working?

THis is driven by the Skin CSS file. You'll have to play around with it.

Leaving a link to your WWW page is always a good idea so we can check it out and make comments.

Edgar.