The most popular host for using images on forums is Photobucket.com. There are a million others, though. I like tinypic.com for no-hassle quick uploads when I don't care what happens to the pic. aww-kittah-aww.com is less hassle than Photobucket and doesn't have rules; you can upload anything and they won't delete it (IIRC, you can use it anonymously or create an account). imageshack.us has been popular since the beginning of time, but I recently found out that it's got pop-ups and other shady crap.
I use PicasaWeb, which suffers the same problem as Blogger: You can't just get an image's address and hotlink to it. You have to change the address manually to add a size specification in pixels of width, and it has to be one of the sizes they support. A URL for a picture might be
http://lh3.ggpht.com/_oNsRR_T1Qx0/SS...8/IMG_2115.JPG
when viewed in the album, but you have to add "s800/" just before the filename to get it to show scaled in 800x600 (or "s640/" for 640x480, "s288/", etc):
http://lh3.ggpht.com/_oNsRR_T1Qx0/SS...0/IMG_2115.JPG
I think these are the sizes that PicasaWeb and Blogger support: 200, 288, 320, 400, 512, 576, 640, 720, 800
It's also possible that I'm wrong and Blogger hotlinking does NOT work the same way as PicasaWeb, but I'm pretty sure it works the same way. If not, it's similar.
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