You can crop pictures by selecting them, then clicking the Crop icon.
You then get this window where you must type how much off the top, bottom, and sides you want.
If you'd rather draw where the crop goes, you can use this extension.
Note, before you begin, that there is no undo.
1. Install the extension. To do this, download it from the link above, then choose Tools > Extension Manager. Click Add and select the extension file. Then restart OpenOffice.org.
2. Create a new drawing file and insert a picture, or go to a Draw file with a picture in it.
3. Select the picture and choose Tools > Addons > CropOOo.