How do I convert a bunch of PNG images to jpeg images at 50% of their original dimensions? I'm on an OS X computer and have already installed ImageMagick.
You can use the 'mogrify' command that comes with ImageMagick in the Terminal. Note however, that mogrify has the ability to silently overwrite images, so *be careful*! In the following example a destination folder is created to save images in order to preserve the original images.
$ mkdir ./results $ mogrify -path ./results -resize 50x50% -quality 80 -format jpg *.PNG
So what did all of that mean?
The first part, "mkdir ./results" created a sub-folder named 'results' to save all of the converted images. You can make a different sub-folder if you already have a folder named 'results' to avoid overwriting contents in that existing sub-folder. Remember to change the folder in the -path portion of the mogrify command, too.
The next part batch converts all of the .PNG images to .jpg images of half the width and height. Note that the extensions are case-sensitive. The "-path ./results" flag indicates that results of the conversion are written to the "./results" folder. All destination images are overwritten if they already exist. The "-resize 50x50%" portion shrinks the final width and height to 50% of the original. The "-quality 80" indicates the compression level of the resulting jpeg images. The "-format jpg" indicates that the final image format should be jpeg. Finally, the "*.PNG" indicates that the input images will be every .PNG image in the current folder.