Big Medium can automatically generate several differently sized versions of your images when you upload them.
How can I get Big Medium to size my images automatically?
Big Medium can automatically generate several different size versions of your images when you upload them (thumbnail, x-small, small, medium, large and x-large), but this feature requires that your server have one of the following graphics libraries installed:
If any of these libraries are installed Big Medium will automatically display a single field for uploading any images and will resize your uploaded images automatically. If these libraries are not installed, you can still upload various versions of the image to the server, but you have to do it manually; you'll be presented with a separate field for each supported image size.
All of these libraries must be installed by a server administrator, so unless you maintain your own server, your hosting company has to do this for you. ImageMagick is, in particular, a very commonly offered library in hosting plans. Contact your hosting company for details.
If you maintain your own server, you can get information about downloading and installing the necessary binaries at the support sites for each of the libraries.