Hello,
I have a large image. I use a smaller version (width: 50%) on some parts of the site. I’m going to create a smaller version to avoid rubbish download. But for the sake of it: why is this smaller version looking good in Firefox and bad in Opera?
Yeah. rh 
Browsers have never been great at resizing images on-the-fly. They’ve got a bit better recently, but still not all that good. That’s not what they’re designed for. Graphics applications are good at resizing images, because they are designed for it.
Two reasons why you should always get your images to the size you want them in a proper graphics application and then upload those to your website:
Most people use Firefox and IE weather Opera is used very rarely. I would suggest you to ignore this issue and work ahead.
What bad advice. Getting thye browser to resize the image is extremely bad even when the browser does do it properly. Its like forcing all your visitors to pay $10 something worth 5 cents. All because you are lazy and can’t be bothered to resize the image properly.
So please do ignore the issue if you want to drive all your visitors away because your pages take way too long to load. Or better yet if that is your goal then simply delete the site completely so as to drive them away faster.
Also since Opera users are intelligent and generally have lots of friends it will soon become well known that you are too lazy to resize images properly even if they don’t make the page load waaaaaaay slower.
Seconding Stephen here. It’s never good to ignore Opera. It’s one of the best and most standards-compliant browsers out there, and if it’s misfiring in Opera, chances are it’s having trouble elsewhere. Besides, as Stephen says, the Opera user community is very savvy.