"Leveling" is one way to get rid of the dirt in the black and white areas. Most scans that come to you will look like a gray film is pasted on top of them. I recommend leveling before the image is resized -- in some cases, this can make the world of difference as to how the image comes out. To level an image, open the leveling box by going to Menu Bar>Image>Adjustments>Levels... or simply using the shortcut ctrl+L.
As you see, the box is a little intimidating. However, all you ever have to touch is the "level black arrow" and "level white arrow". To erase the dirt from the black area (that is, make the black blacker) drag the "level black arrow" to the right. To level the white area, drag the "level white arrow" to the left. As you drag either arrow, you'll notice the numbers in the value boxes up top will change. If a person says: "I leveled the image 237 white," it means they leveled the white area until the "level white value" reached 237. Here is an example of the above image leveled:
Notice how the final, leveled image looks very crisp? That's what you're aiming for. Also, notice in the upper left corner that there is still dirt. This is called "gutter shadow", and you level it separately from the rest of the image. Do not level the entire image until the gutter shadow disapears. If you did this, you'd be "over-leveling", which is where you lose midtones (the gray hues on the page). For example, if I leveled this image until the gutter disappears, the final image would look like this:
See how it looks burnt? And the midtones on the shirt, the ground, the trees, and just about everywhere have lightened and look choppy? This is an extreme, obvious example of over-leveling. In order to avoid any over-leveling, I recommend that you level just until the areas are free of dirt.
If you're worried about overleveling an image, there's always the option to create an adjustment layer which will level the layer(s) below it. To do this, at the bottom of the Layers pallete, choose "Create new fill or adjustment layer" -> "Levels...". You'll then be prompted with your standard levels box. Once you set the levels, a new layer will appear in the pallete, and all layers below it will be leveled to the specifications you entered. Double-clicking the layer thumbnail on the adjustment layer allows you to change the levels at any time (see all this). Be careful to put all the text layers above the levels layer, or they'll be leveled and look choppy.
That takes care of levels. Read on to see how to get rid of that pesky gutter shadow.
Continue to Gutter Shadow ->