You want to set a constraint function:
from zope.interface import Invalid
from foo.bar import MessageFactory as _
def imageSizeConstraint(value):
# value implements the plone.namedfile.interfaces.INamedBlobImageField interface
width, height = value.getImageSize()
if width > 500 or height > 500:
raise Invalid(_(u"Your image is too large"))
then set that function as the constraint
of your NamedBlobImage
field:
image = NamedBlobImage(
title=_(u'Lead Image'),
description=_(u"Upload a Image of Size 230x230."),
constraint=imageSizeConstraint,
required=True,
)
See the Dexterity manual on validation for more information, as well as the plone.namedfile
interface definitions.