Which vaultwarden version and web-vault version are you using? The examples from the wiki seem to work for me.
(You might have to mount the images into the /web-vault/images directory so that you can reference them via url(../images/my-custom-global-logo.png). Vaultwarden’s CSP might prevent loading the image if you reference external images.)
btw: the SVGs are now embedded into the compiled application so they cannot easily be replaced by changing custom images in the web-vault images folder. You’d need to replace the whole <svg>...</svg> images inside the compiled javascript which is not something I would recommend.