Before this commit placeholder images were generated in multiple different ways in website_slide. This commit makes use of _get_placeholder_filename function and standardizes the way these placeholder images are generated for website_slide. Task-2908029 closes odoo/odoo#100587 Related: odoo/upgrade#4050 Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>