I Build Joomla! web sites since 2008, and have tried half a dozen extensions that handle this job: Auto-create thumbnails, straight from the images authors add to their articles. Also helping us to get a nice layout on blog pages.
jdvthumbs does that well, and offers different settings (e.g. sizes) for Featured articles, Blog Pages, Articles and more.
It can also add a watermark on all your images in seconds.
And basic functions are very easy to configure.
Cons (possible improvements):
Support: well, i found a form on the author's website and after translating it from Russian I sent them a message about the PHP Warning described below. Am waiting for a reaction.
It rename the rezized images from their PATH. i.e. an image that shows up on 5 (blog) pages will have 5 differents thumbnails, adding as many files and therefore, requests on the server.
Watermark does not handle PNG alpha imgs
Beware of some Rezise settings, I've found out some have awful results, like "source": it can multiply the image's size by a factor of 40x!
e.g., a png image, original is: 438x438 px, 6.29 KB
In jdvthumbs settings
- thumbnail is 199x199 px and 248 KB (!) with Method: PNG's alpha-channel: Yes; Thumbnail's Type: source; Area for cropping images: Middle.
- Or 199x199 px, 3.65 KB with thumbnail's Type jpg.
Note that Smush.it resizes the same image at 2.6 KB while keeping it PNG.
If on a PHP 5.3+ server with jdvthumbs you ever get a warning (Warning: Parameter 3 to plgContentJdvthumbs onPrepareContent). Just edit jdvthumbs.php's syntax to be PHP 5.3 compliant and the Warning is gone. (this has countless references on the Web)