The Imsanity plugin for WordPress takes oversize photos and scales them to a more suitable size for display on a website. While there are several other ways this problem can be tackled, Imsanity provides a simple and reliable option.
One option for resizing images is to set the three media sizes under Settings > Media. You can add extra resized versions by using WordPress's add_image_size function. You can find more about that here. But that means messing with your functions.php file. And then to actually generate those new sizes on images already in WordPress you'll need something like the Regenerate Thumbnails plugin. You can also run into other quirks with this. I find that often the new sizes are not available for selection in the WordPress dashboard embed features. And none of these options changes the original file--it just applies to resized derivatives. If you have users uploading high resolution images directly from a camera, WordPress's built-in features won't shrink them.
The most basic way to tackle the problem is of course to resize the images before uploading. But that adds extra steps, and if you're running a site with other users, not every user will know how to resize reliably.
The Imsanity plugin takes out the guesswork, simplifies the workflow, and saves you from ending up with enormous images in WordPress hogging your bandwidth and slowing down your backups. It's designed to resize the original master images uploaded to WordPress. It does not serve as a replacement for resizing derivative versions.
You install the Imsanity plugin the usual way. The settings/configuration page is under Settings > Imsanity.
There's not much to configure except to set the maximum image size you want and the JPG quality. The Convert BMP to JPG option is especially useful if users are posting screenshots.
If you're installing this plugin, there's a good chance you've already run into the problem of having huge images in WordPress and are looking to resize exisitng images. That's where the Bulk Resize tool comes in. To access it, just scroll to the bottom of the Imsanity settings page (Settings > Imsanity).
There's only one button, but there are some warnings to take note of. One is that there's a limit of 250 images per run in order to prevent the server from coming to a grinding halt. The other is that resizing is irreversible. Read the warning in the red box before proceeding.
After you hit the Search Images button you'll get a list of the images that the plugin has proposed for resizing. You can check or uncheck images individually.