Most people confuse DESIGN and DEVELOPMENT. So are the previous answerer.
Development is "programming": make the images respond to user's actions. It is the hard and hidden part of the web. 99% of people THINK it is easy, because of all these "web-maker" programs that flog the web, and that a total beginner CAN make a page in a couple of hours. Making a complex website is far from simple, and it is not rare that the coding of a simple e-commerce can take several weeks of hard work by SKILLED programmers!
Design is "art", and, for the web, you must add "marketing": a badly designed site does not attract buyers...
Marketing can be studied. Art can be studied. Photoshop makes great web-pages that a good programmer can use. The designer does NOT need to know anything about HTML or CSS: that is the role of the programmer. In fact, I have never known a good designer who could put two lines of HTML together correctly! I DEMAND source designs in layered Photoshop.
BUT... does the knowledge of all this make you a GOOD designer? The answer is "no".
You may know how to use every colour of the palette, every technique, every program (I personally know them all), that does NOT make you a Salvador Dali or a Picasso (! am absolutely crap!)...
It is not hard at all... IF (BIG, BIG "IF") you have the GIFT to make images that have the "WOW" factor.
And that is given to one person in a million...
http://web2coders.com - professional developers for professional buyers. Many apply, very few qualify.