Open the image you want to crop in Illustrator. You can drag and drop the image file onto the Illustrator workspace or open it using the File > Open menu.
Select the image using the Selection Tool in the left tool tray. It looks like a black cursor with a white outline.
Using the Selection Tool, click on the image you want to crop. When it's highlighted, a box will appear around the edge.
To crop the image, use the Crop Tool. Go to Object > Crop Image. With the crop tool, you can adjust the image by dragging and dropping the edges.