Here are three forms with the same actions but differrent types of submit. Observe what it does in differrent browsers.

1. BUTTON type="submit"
2. INPUT type="image"
3. INPUT type="submit"

Results

IE6 FF2
a 1 BtS=Text+in+the+button+type%3D%22submit%22 BtS=value_of_the_button_type_submit
2 InTI.x=5
InTI.y=5
InTI.x=5
InTI.y=5
InTI=value_of_the_input_type_image
3 InTS=value_of_the_input_type_submit InTS=value_of_the_input_type_submit
b 1 BtS=Text+in+the+button+type%3D%22submit%22
othS=othSValue
othS=othSValue
2 othS=othSValue othS=othSValue
3 othS=othSValue othS=othSValue

IE7: in b1 behaves like FF

Robert Nyman: "Not all values posted with a form in IE"