This function is available for newer versions of TYPO3 (Pixelant Core).
Each image has a tab called "Cropping". On that tab you will find a dropdown where it says "Crop disabled". Select the ratio you wish to use. (Img. 5.4.1)
Go to the tab "Cropping" again, where the image has now been loaded below the dropdown. Select the part of the image, you wish to use, by dragging and dropping inside the loaded image. You will see that it keeps the ratio you selected. Adjust by dragging the corners. You can also move the area, by clicking inside the selected area and dragging. (Img. 5.4.2)