you don't need the double = in the line... its actually bad... also you need
to use ()'s so the parser knows what attrabutes go to which helper...   Your
first example is actually sending the "img" helper two arguments, the image
and the url and assigning them to link.  You actually want to send the
output of the image helper and the url to the link helper.

This should work:
= link( img("question_sense_logo.png"), "http://www.questionsense.com")
which is the same as:
= link img("question_sense_logo.png"), "http://www.questionsense.com"

I didn't find an API on what the link helper is expecting but it looks like
the first attribute is the innerHTML of the a tag and the second parameter
is the href.  It is possible that the link helper does not support the
output of the img helper as the first argument but I doubt it.

If that doesn't work then this will work:
%a{:href=>"http://www.questionsense.com"}= img("question_sense_logo.png")

For good measure you should add alt and title attributes to your anchor tag.
%a{:href=>"http://www.questionsense.com", :alt=>"Question Sense",
:title=>"QuestionSense"}= img("question_sense_logo.png")

(These attributes will be friendly with blind viewer programs and give the
browser something to display if the image is not avilable etc... Also I
believe the html validator at W3C <http://validator.w3.org/> will not pass
you without an alt attribute.)

To help your understanding of whats going on with the haml parcer:
= tells the remainder of the line to be evaluated as ruby code and the
output replaces the line (useful for variables etc)
- tells the remainder of the line to be evaluated as ruby code with no
output  (useful looping, variable assignments, etc)
%tag_name{} (where tag can be something like "a" "div" etc) the tag and
following optional hash is evaluated as ruby code.  The remainder of the
line will be considered text unless you have an = after the tag_name in
which case the remainder of the line is evaluated

These three examples would output the same thing

1)
  Hello World
2)
  = "Hello World"
3)
  - some_variable="Hello World"
  = some_variable

Note in the third example that there is an = sign... this is used as an
assignment which is the useual use of = in ruby.
The following would also output hello world but is kind of silly... I am
using it only to show you what the second equal sign does:
4)
  = some_variable = "Hello World"

This line sets 'some_variable' to the value of 'Hello World' and outputs the
value of 'some_variable' as text to the page from the first equal sign.  It
is exactly the same as option three but done in one line.

It will be worth your time to read up on "haml"

"No man making a profession of faith ought to sin, nor one possessed of love
to hate his brother. For He that said, “Thou shalt love the Lord thy God,”
 said also, “and thy neighbor as thyself.”  Those that profess themselves to
be Christ’s are known not only by what they say, but by what they practice.
“For the tree is known by its fruit.”" -- Ignatius