how to delete width and height attributes from image field for particular nodes I would like to strip width and height attributes from image fields for a few specific nodes. 

I tried hook_preprocess_image($vars) but $vars don't have enough information. From the documentation, theme_image() won't have that information, either. I can't figure out the node from the path, because the node is used in a block. Using CSS width:auto !important; does not work, at least with Firefox 16 (and I've read IE 8 has problems with it, too). Any ideas?
You may be able to accomplish this with view modes.  

Make some new image styles for these particular images.  Implement a theme_image_style(), which checks for this new style name and unsets width and height as needed.

If you install/enable Entity View Modes you can create new view modes for your nodes.  Clone what you have for node/teaser to your additional layouts.  For the images, change the display settings to use your new styles.

Next is the part I am not sure what the best solution is.  One way to use the view mode is to use a Panel page for each individual node.  When you add the content, you can set the view mode to one of your new ones.   Otherwise, I would try @PierreBuyle's solution at Using a different view mode with a node.  This essentially overrides the menu page for node/nid to add some additional intermediate logic to set the view mode.