Bonjour Nick
I quickly checked:
From flickr one can't get an image (I mean the real file wich is host on the web server) as it is protected by a gif layer (named spaceball.gif), but I could get one (n27705755_32110980_1532.jpg) and found that there is no icc (or icm) profile embedded within the image (sRGB should be the one for the web).
So each browser will freely interpret the colors of your image. Safari has been the first browser to be ICC aware (it shows the image according to the embbeded profile) many other do also now.
Hence the great importance to embed the profile you intend to be used. As said, the one for the web or onscreen viewing (exceptf for the happy few who own Adobe RGB complant high-end monitors such as Eizo).
I don't know how flickr or facebook do handle this, but I doubt that they remove the embeded profiles…
For the format of files, jpegs saved with 8 (on PS scale from 1 to 12) is generally a godd choice.
Nowadays, the smaller screen used to browse the web are 1024x768 pixels so for posting landcape format you may easily go up to 800 pix and 700 in height for portrait oriented images.
Posterization? choose any image, open in PS, do a "save for the web" then choose gif export and look the results when you downsize the # of colors… you'll see what posterization is!!!
As to the size in Kb of the file shown, one must be carefull not to upload to heavy files, even if now most of people have a fast connecion. Below, for info is how peope do connect to OPF for a certain period of time:
Screen size:
1. 1024x768 22,45 %
2. 1280x1024 18,19 %
3. 1680x1050 12,70 %
4. 1280x800 10,56 %
5. 1920x1200 8,14 %
6. 1440x900 8,13 %
7. 1600x1200 6,16 %
8. 1152x864 2,07 %
9. 800x600 1,85 %
Speed connection
DSL 10 839 32,47 %
Cable 10 419 31,22 %
Unknown 8 495 25,45 % (most of unknown are "corporate" network… fast!
T1 2 400 7,19 %
Dialup 989 2,96 %
Browser Visits (in %)
I Explorer 14 543 43,57 %
Firefox 12 490 37,42 %
Safari 5 257 15,75 %
Opera 671 2,01 %
Camino 173 0,52 %
visitors come from
1. United States 17 216
2. United Kingdom 3 398
3. Canada 2 284
4. Australia 964
5. Netherlands 877
6. Germany 854
7. France 627
8. Spain 443
9. Italy 382
10. Switzerland 360
Hope it helps