As requested in https://bugs.tlgr.org/c/4360/6 this is a ticket for Telegram Desktop (Windows). Webp images behave like stickers, not like other images. They do not appear in the image gallery. They cannot be sent as a "bulk" (multiple selected webp images show as separate messages, unlike multiple simultaneous jpg-s).
The upper and lower image in the screenshot are both .webp images. The upper image behaves 100% like a sticker (no interaction possible), the lower CAN be clicked/fullscreened, but is not part of the image gallery also.
Send an .webp image from your device.
Telegram Desktop 2.9.2 x64, PC 64bit, Windows 10