Hi
The mobile logo images were removed with version 3.9.0. The other images were moved from "paypal/img" to "paypal/views/img", but the mobile logos just got deleted and never replaced.
You can still find the images here: https://github.com/PrestaShop/paypal/tree/c3f7abaa591a09d52a57c959bb07c1ec00a5131e/img/logos/express_checkout_mobile
If you add them to the new img location it will work.