To be able to use a program as a command, add the folder containing the .exe to your Path variable, which can be found in Advanced System Settings ⇒ Environment Variables. Reopen any consoles.
Download the portable version of ImageMagick and put the file magick.exe in a folder that is in your Path variable.
Make a .bat file with the following contents:
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.
Image files such as JPEGs can contain privacy-sensitive information, such as GPS coordinates, which tell where the picture was taken. Such information can be viewed and edited using the stand-alone ExifTool Windows Executable. To just remove all metadata, put
magick mogrify -strip %1
in a .bat file. Drag an image file to it before uploading to a public place. Sometimes color information is also stored as metadata, which will also be removed.
Download the Xpdf command line tools and use
pdfimages -j file.pdf outputname
to extract the images from a PDF as JPEG, PPM and PGM files.