Buying Quick $5: Jquery/Javascript

Ok, I need someone to write some code for submit button to be disabled until a $var is empty... aka if(!empty($errors)) { enable submit button } .... I also need a similar script that disables submit button until either an image is uploaded or checkbox is checked

Here is the submit button that needs to be disabled

	 <input type="submit" name="next" class='next_btn' value="Next"  />

Code (markup):

 

PM with code sent. I'll change it if you want something different.