Loading images according to screen width
I have a few comments, which in no way substitute a full review.

Your indentation is inconsistent which makes your code unnecessarily hard to read.

Your comments are also inconsistent. I'm not going to prescribe you a commenting standard, but pick a style you like and stick with it.

Now we're talking about comments, the following is redundant:

var thisImage=images[i];  // Grab Current Image // 


It's still clear what you're doing without the comment. It's commenting the 'what' of your code while you should only be commenting the 'why'. The 'what' is already obvious in this case, as it should be. You may be interested in reading Writing Readable Code, about readable code and what part comments play in this. Comments can be good, but they are often not necessary.

You're only using the width of the resolution. While most desktop screens use standard ratios, you seem to target mobile users. Mobile devices don't necessarily stick to standard ratios. For width your screen may be a small, but for height it could be a medium.