How can I post buttons and images links in my sidebar?

If you want to display a Del.icio.us or Bloglines button for any image link, you can do this from Links Management.

Click Write -> Link from your Dashboard. If you scroll down, there’s a box for you to enter an image url. Fill that in, and an image will be displayed instead of the link text. The image doesn’t even have to be hosted here either.

You can also use a text widget and link directly to an image.
To find the url of an image that you have uploaded, check How do I manage my uploads?

You can also look at ‘Badges’ at the top of this blog.