What is WEBP?
WebP is Google's image format for the web, and the rare one that does both jobs: lossy compression that beats JPEG at the same quality, lossless compression that beats PNG, and an alpha channel either way. Every current browser reads it; older software often still does not.
What is TGA?
Truevision TGA is a fixture of game development and 3D texturing, where its straightforward layout and real alpha channel have kept it in use long after browsers stopped caring about it. Files may be raw or run-length encoded.