Download the portable version of ImageMagick and put the file magick.exe in a folder utils on your Desktop. Make a .bat file with the following contents:
utils\magick %1 "%~dpn1.jpg"
You can drag WEBP files to this batch script and they will be converted to JPEG. Other image formats work too. For SVGs you need to install or unpack Inkscape and add the path to inkscape.exe to your Path variable, which can be found in Advanced System Settings ⇒ Environment Variables.
Download the Xpdf command line tools and use
pdfimages.exe -j file.pdf outputname
to extract the images from a PDF as JPEG, PPM and PGM files.