How do I crop a photo in Illustrator?

Clipping paths and masks can be temporarily applied, and then released from release (accessible in the objects panel).  This can restrict parts of the image for viewing, or make them viewable again layer.   Here is a link to a video explanation: click here

1. Go to view --> show edges or use  (control H or cmd H)
2. Press V on the keyboard (brings up selection tool), then click the picture you are wanting to crop
3. Somewhere on the screen (probably top), a mask button appears on top of the control panel
4. Click the Mask button (in that control panel) --> creates a clipping mask

Two buttons at the far left of the same panel that had clipping mask button show up (far most left-two buttons).  after you click the first of those buttons, use (keyboard a shortcut) to start clicking anchor points at the far-most corners of the image

• After selecting topmost anchor points, use shortcut shift down. 
• You can select any edge (made of at least two of those anchor points (and use shift key + UP DOWN LEFT RIGHT aft)

ANOTHER TRICK= OPACITY MASK

• Use the shape drawing tools (i.e. ellipse tool) --> then use a white fill color (optimally)
• Create white circle atop the car (or above the vector artwork)
• Use V key to select the white ellipse, then shift select the car to select the image behind the ellipse, and then open the transparency option (window panel ==> transparency panel)
• Within the transparency panel, expanding the menu that appears, and select the make mask button, which allows you to see ONLY WHAT YOU HAVE IN THE FILLED SHAPE or ellipse (as shown in the video linked at the top).
• you can adjust the mask or the artwork you are trying to restrict and also, you can release any of these clipping masks

Advantages over cropping:

• In the video, you can actually modulate some property of the clipping mask (say add a gradient and opacity mask), which actually saves you some time by allowing you to not only crop, but change the entire image, and making it easy to release that mask so your raw materials are usually in tact.