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8-bit, sRGB is the colour standard for the web.
I go a step further and include the colour profile just in case the viewer has the interest to have a calibrated and profiled monitor and also a profile-aware web browser. Of course, this also implies that your own screen is calibrated and profiled. This is especially important if you post images for feedback because you want to do your best to ensure that the viewers see what you saw.
Typical screens these days offer 100-110ppi screens but a few high-res screens offer 200-300ppi (Apple retina screens, MS Surface Pro, etc.). I don't think you'll find a screen that offers 72ppi these days so don't use that, but catering for retina screens is probably bad form except on your own web site or one dedicated to high-res screens.
Width is often determined for you by the site to which you send the images (e.g. here at FM it is something like 800 pixels but that is often too small). Don't forget that you can post cropped versions as separate images to show important details that are not visible in the full shot at low res. If it is your own site then the main limitation is the expected download/upload costs and how long it takes.
In terms of sharpening there are two schools of thought: For general viewing sharpen to suit the viewers' output devices - i.e. as if it will be viewed on their monitors in a web browser. For technical comparisons do not sharpen at all - let the viewer download the images and apply their own sharpening so that their evaluations are not spoiled by what you have already done to the images.
important: Make sure that you include all relevant EXIF info with each image. It helps others determine what you used and how that could have been improved, for both your benefit and theirs.
Edited on Mar 20, 2013 at 10:33 AM · View previous versions