Hello I have been asked to make a post about Images. After in depth discussion with several department heads and some faculty and several years of diagnostic studies my findings are 'A picture is worth 1000 words.' 
       Did you know that someone actually had the audacity to suggest that this assessment has been arrived at before? 
           All humour aside pictures can be very effective pedagogy when used in there proper context. Biology is a particularly pectoral subject. A lot of it is route learning, but without the pictures it would really not be possible without actual various dead creatures in the class. 
         To some extent this can be done. However, you are still using the graphic representation to get your point across. You won't understand the circulatory system of any animal (to include human persons) unless you see a circulatory system. Now you will probably get the idea of how the human body blood system functions by seeing how a frogs system functions and making the necessary mental adjustments. So long as you observe enough pictures of human systems to understand the differences. 
       Chemistry is the same way. You will not really understand molecules until you see them. Physical models are very expensive. Software is cheep and everyone has a computer they can make there own molecules. However, its more than just seeing them. If you do not understand how important shape is to an Enzyme and its function in the body you will never understand an Enzyme. There are concepts like chirality that are vitally important that will never be understood if they are not seen. 
       Your hands are Chiral.
 

 Source flicker retrieved 31/03/15 : https://www.flickr.com/photos/nebneb/3693520916

 
 Both left and right have fingers, thumb, palm, and back, but neither can replace the other. Simply put chirality is moleculor 'handedness'. Without creating that mental picture of the hands or showing you pictures of molecules (lots of them) you would not understand what I am talking about.  

      To put it in the SAMR model. Images can be a simple substitution, as in biology, allowing what would not really be possible to teach, economically, now to be taught to everyone (try teaching human anatomy with the real thing, even if you follow all the rules the expense is every great).
        In the example above I am Modifying the discussion. Instead of drawing examples of molecules I am pulling an image everyone understands straight off the creative commons and bringing it right into the class to explain simply what otherwise may be difficult to understand. 
     However, chemistry modelling software (free in some cases) has completely Redesigned the idea of teaching chemistry. (Pause for angelic AHHH) No one has to buy really expensive molecular stick models for class room props. Why would you do that?

  Now I have an interactive white board. 

        Surly it is not that hard to use this software to create a molecule that can be manipulated in real time in the classroom. Molecule designs are commonly traded between researchers, educators, etc.. or downloaded from online. Students can see for free what I cannot show them because it is too small. 
          This sort of software also allows the assignment 'Create a model of a hydrocarbon in common use'. 
Talk about redefining the learning experience.