The Blur Tool unfocuses image areas: In the Toolbox, select the Blur Tool. Choose brush size and style*. Set the tool's strength. Drag on the image. on the Options bar you can choose also effect mode.

In this tutorial Scott shares some tips for using the elliptical marquee tool and demonstrates how to use layer masks.

Add Anchor Point Tool adds anchors and reshapes existing vector shapes / paths (shape outlines).
In the Toolbox, select the Add Anchor Point Tool. Click on the existing path to add an anchor point. Drag the added anchor point and / or it's direction point to reshape line...

As you already know, the optimization is a search for the best image quality / file size balance. We have to remember about visitors with slower Internet connection.

Let's look at the Toolbox once again. As you can see it is full of some (supposedly useful) instruments. And there are tools that we can't even see yet! Some of these buttons contain hidden tools. Press any button that has a small arrow in the corner, to select a hidden tool...

Contrast in photography is the difference between light and dark areas of an image. By increasing the contrast, you can bring out more detail in a photograph, and make an image appear much more vivid with color.

If you find yourself needing to quickly remove the background from an image in Photoshop, take a moment to play with the background eraser tool.

Use brush on old fashioned, real white paper to make some more or less straight lines. Scan them in gray or B&W mode on 300 pix/inch resolution.
You should get an image that looks like this:

Here is a quick introduction to channels (which are covered more in depth in my video training with Basic Photoshop). In the Channels palette you basically have your different color 'channels'. I'm still waiting for the MTV channel...

A less well known feature of Photoshop is that you can create custom vector shapes and use them throughout your designs. Here's an of examples of how to do it.