You are using the correct way to define an image as the submit button.
Just exactly what is happening that indicates that it is not working.
Is the image displaying? If not then the path to the image is wrong.
If you click on the image then the form will be submitted - if it isn't then one of the validation tests that you are doing must be wrong.
It is usually best to not name submit buttons/images at all as the information passed is not useful - particularly when using images where both followbutton.x and followbutton.y will be zero if the form is submitted from the keyboard and will only contain values indicating a position within the image if the form is submitted by clicking on the image.