[FIX] website_slides: use image_1920 instead of invalid 'image' field

Since the renaming of image fields to image_resolution (image_1920 in our case)
slide upload widget is using invalid field.
This commit adapt the field name to use the new naming (image_1920) instead.

Task id: 2179219
PR #43663

closes odoo/odoo#43816

X-original-commit: 5991eb5b
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>