Ok, here's what I do if I ever need to crop out excess parts of an image. I especially like to use this for large images in order to reduce file size. Make a rectangle to the size that you want the final cropped image to be. Place this rectangle over the image to mark the cropping area. Fill the box with a color, any color will do. Open the Transparency Pallette and change the transparency of the colored rectangle to 0%. Select both the transparent rectangle and the image. Click on Object in the menu bar, then click Flatten Transparency. Change the Preset to High Resolution. Click ok. Now, release the clipping mask and ungroup the image. You should now be able to select the out portion (the part being cropped out) and delete it. There will also be a couple rectangles that have to stroke or fill color that you may want to go ahead and delete also. If there are multiple rectangles leftover that make up the image, you can always select these and group them. I hope that didn't sound too confusing. I do it so much that it's almost second nature to me. If you need more help let me know.
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