Filenames

Keep the file names lower case, with hyphens between the words:

this-is-my-picture.png

The rationale is as follows:

Having everything in lower case add security in all situations where you are to type the file name (referring to the file from a different source file, HTML++) as you always know the case of all the words.
Lower case file names are easier on the eyes. When looking at files for hours on end, having them in lower case easens the strain on the eyes as:

MiXEDCaseWordsAreConfusing_and_can_STRAIN-youreyes.doc

In stead, consider this file name:

mixed-case-words-are-confusing-and-can-strain-your-eyes.doc

Lower case is faster to type as you don't have to press shift or caps lock to get upper case letters.
Hyphens are easier to type than underscores as you don't have to press another key to get the character.

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