Imagemagick is a very powerful ‘Command Line’ program, available for Windows,Linux and macOS to convert images from one format to another, resize them, and a whole range of other functions more specialist than those covered here.
As a command line program imagemagick is used from a Command Window under Windows or a terminal program under Linux. Under Windows it may be that the commands shown need to be prefixed with ‘imagemagick’
Resizing Image Files
WordPress websites normally a maximum file size for images they will accept for an upload. Modern digital cameras will often create images much larger than are typically accepted. In most cases having huge images on web sites does not add much to the viewing experience, so it makes sense to convert images to a smaller size before upload.
For an individual file you should use something like:
convert inputfile.jpg -resize 800x600 outputfile.jpgIf you have a number of files to resize you should copy your originals to another directory, change directory to that directory (usually by cd directoryname) and then run something like
mogrify *.jpg -resize 800×600
Converting between file types
If you have an image file in some format, say webp, which the software you want to use with it does not handle, try
convert thisimage.webp thisimage.jpg
and imagemagick will work out from the file types given that it should convert thisimage to a JPEG.