Although "dog. jpg" and "dog. jpg" and "dog. jpg" and "dog. jpg" all appear to be the same file, they are not considered the same when programming web pages on the internet.
why?

Because the capitilazation is different.

HTML is case sensitive. When it matches strings and filenames and all other kinds of things (except for tags) it checks to see if the cases match too. If they don't, they're not the same, and it doesn't return anything. As a beginning programmer, I messed this up a lot! Basically cases aren't interchangeable in HTML, so you have to be consistent.

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