[web2py] autodelete image computed field

[CtrlSoft]

hi i have a resize function wich creates image thumbanils...
def THUMB(image, nx=120, ny=120):
    from PIL import Image 
    import os  
    try:
        img = Image.open(request.folder + 'static/img_folder/' + image)
        img.thumbnail((nx,ny), Image.ANTIALIAS) 
        root,ext = os.path.splitext(image)
        thumb='%s_thumb%s' %(root, ext)
        img.save(request.folder + 'static/img_folder/' + thumb)
        return thumb
    except: 
        return None  
 
and this :
db.define_table("news",
    Field('image', 'upload', requires=IS_EMPTY_OR(IS_IMAGE()), 
uploadfolder=request.folder+'static/img_folder', autodelete=True), 
    Field('image_thumb', 'upload', compute=lambda r: THUMB(r['image']), 
autodelete=True), 
    Field("title",label=T('Title'),requires=IS_NOT_EMPTY()),
    Field("content" 
,type='text',label=T('Content'),requires=IS_NOT_EMPTY()))

the problem is when i delete a row, image("big image") is deleteing from 
disk but the image_thumb stil remains on disk. 
any solutions are welcome :), thx

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Alex