Use Preview’s Instant Alpha feature to erase backgrounds in images or make certain parts transparent.
Start with a base image. In this example, I use a picture of the Minneapolis skyline and will try to remove the fog/clouds on top of the building.
Open your image in Preview. Click the Edit button and then the Instant Alpha button.
Drag and hold your cursor over the clouds. As you move your mouse (without letting go), you will see the red area increase/decrease. This colored area will be what will become transparent.
Depending on your image, you might go too far and highlight something you don’t want. In that case, just reverse your mouse.
Press Delete to remove the clouds. You will be prompted to convert the file to a PNG since that format supports the alpha channel.
Now the clouds will be deleted. Depending on your image, the feature might not have grabbed everything. For instance, on one of the buildings, I zoomed in and refined the part that was missed.
Now the image will be transparent where the clouds used to be. You can easily use this feature to customize logo on the OS X login screen, or use it with Photoshop to make custom images.