It's not very hard, here's a possibility that requires you to have pillow 
<https://pillow.readthedocs.org/> installed.


auth.settings.extra_fields['auth_user'] = [
    Field('photo', 'upload', label=T('Photo'), uploadseparate=True, requires
=[IS_EMPTY_OR(IS_IMAGE(error_message=T('This is not an image.'))), IS_LENGTH
(maxsize=1048576)]),
    Field('avatar','upload', uploadseparate=True, readable=False, writable=
False, autodelete=True, compute=lambda row: SMARTHUMB('auth_user', row.photo
, (32, 32), name='avatar')),
]



def upload_fullpath(tablename, filename, makedirs=False):
    """
    Given a tablename and a filename return the full path
    TODO: For now it assumes uploadseparate=True in the field definition,
          could be more versatile.
    """
    import os
    from gluon import current
    fieldname, img_uid = filename.split('.')[1:3]
    path = os.path.join(current.request.folder, 'uploads', tablename + '.' + 
fieldname, img_uid[:2], filename)
    if makedirs:
        try:
            os.makedirs(os.path.dirname(path))
        except OSError:
            print "Skipping creation of %s because it exists already." % os.
path.dirname(path)
    return path


def SMARTHUMB(tablename, image, box, fit=True, name="thumbnail"):
    """
    Downsample the image.
    
http://www.web2pyslices.com/slice/show/1522/generate-a-thumbnail-that-fits-in-a-box

    Modified to respect uploadseparate=True differences

     @param img: Image -  an Image-object
     @param box: tuple(x, y) - the bounding box of the result image
     @param fit: boolean - crop the image to fill the box
    """
    if image:
        from PIL import Image
        import os
        from gluon.tools import current, web2py_uuid
        request = current.request
        img = Image.open(upload_fullpath(tablename, image))
        #preresize image with factor 2, 4, 8 and fast algorithm
        factor = 1
        while img.size[0] / factor > 2 * box[0] and img.size[1] * 2 / 
factor > 2 * box[1]:
            factor *= 2
        if factor > 1:
            img.thumbnail((img.size[0] / factor, img.size[1] / factor), 
Image.ANTIALIAS)
 
        #calculate the cropping box and get the cropped part
        if fit:
            x1 = y1 = 0
            x2, y2 = img.size
            wRatio = 1.0 * x2 / box[0]
            hRatio = 1.0 * y2 / box[1]
            if hRatio > wRatio:
                y1 = int(y2 / 2 - box[1] * wRatio / 2)
                y2 = int(y2 / 2 + box[1] * wRatio / 2)
            else:
                x1 = int(x2 / 2 - box[0] * hRatio / 2)
                x2 = int(x2 / 2 + box[0] * hRatio / 2)
            img = img.crop((x1, y1, x2, y2))
 
        #Resize the image with best quality algorithm ANTI-ALIAS
        img = img.resize(box, Image.ANTIALIAS)
 
        root, ext = os.path.splitext(image)
        thumbname = '%s.%s.%s.%s%s' % (tablename, name, web2py_uuid().
replace('-', '')[-16:], web2py_uuid().replace('-', '')[-16:], ext)
        img.save(upload_fullpath(tablename, thumbname, makedirs=True))     
   

        return thumbname



-- 
Resources:
- http://web2py.com
- http://web2py.com/book (Documentation)
- http://github.com/web2py/web2py (Source code)
- https://code.google.com/p/web2py/issues/list (Report Issues)
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