I also registered a while ago, and it seems that when you do that you can upload more at the same time, but I never use that system. So you don't have to register since you can use imageshack anyway.
I use imageshack to upload all my pictures, from avatars to large files. I used it for the first time about a year and a half ago, and those pictures are still online, so I'm pretty sure they'll stay.
When I upload an image I always make sure that it's already resized, and in some cases, watermarked.
At the front page of imageshack you can fill out a lot of different things, but the only thing you have to use is the name of the file. Just write down the filename and click 'host it', you don't have to log in or something.
When you upload a file, you'll see a page with a thumbnail of your file, and a few links. You should only use the last link, the direct link. If you know a little bit of html or bb coding, that's all you'll need. The other links, like 'show to friends', directs you to a page where the image is shown, but you'll get popups and such when you click that link. (Imageshack prefers it if you use the page with the ads, ofcourse)
I normally never use the thumbnail function of imageshack, because clicking a thumbnail also brings you to the page with ads. But if you have large images and want to show a thumbnail anyway, you could also change the link a bit.
In case you're using this forum (or a similar forum), you have to pick the second link on the page (thumbnails for forums 1). Simply delete the link between in the first URL= tag, and replace it with the direct link. This is the result:
You can change all links in this way, ofcourse, and resizing for websites is easy if you use html coding.
After you've uploaded the pic and closed the screen, there's no way to get back to the page with links. (if you're not logged in) But there's a simple way to save pics for later: Just open a notepad (kladblok) or other text file and paste the links in there. I added a tiny description to all my links to tell them apart, so now I have a few hundred images hosted online, which I can use whenever I want, using just 3kb of harddisk space.
Imageshack also works fine if you want to host animated gifs.
http://img181.imageshack.us/img181/5720/sambaox7.gif