Are your images not sharp enough or way too sharp ?
It is actualy super simple to get your images to look good online, a few easy steps and you can have your images crystal clear and correctly displayed over multiple different monitors, phones and pads.
The first step is simple but will cost you a few bucks, get a calibration tool that will analyze the colors on your screen and adjust them correctly.
Its incredible how your mind can play tricks on you, if you have a monitor with too cool colors what is supposed to be white is more a light blue and if you get too used to it the odds are that you will compensate when editing your images and make them a lot warmer than they should be.
Most monitors change over time either becoming too warm or cold in color, using a color calibrator will ensure accurate colors at all times.
I personally use the Spyder 4 pro since it also can analyze ambient light (i.e the light in the room where you are working with the images), and it ensures that my images will be viewed with the colors I intended on other monitors.
There are many different color spaces, like AdobeRGB, Prophoto, CMYK and sRGB.
Where the hickup lies when it comes to color space is not converting it to the proper colors space for the given medium displayed upon.
In 1996 HP and Microsoft created sRGB (the standard RGB color space) for use on monitors, printers and the internet.
Some websites do have algorythm in place to automatically convert a color space to sRGB but most do not, so if you upload an image online for display in for instance AdobeRGB, odds are the colors will look way different than your original image, so converting the image is crucial for accurate display online.
In Photoshop you can go to the Edit menu, scroll down to Convert to profile and you can convert a source space to a destination space, in this case sRGB (Also the save for web feature will automatically convert the color space).
8bit vs 16bit
In short, shoot your images in RAW and edit them in 16bit, only when you finalize an image do you convert them to 8bit, which is most often just the last sharpening.
Sharpening for the web
When I started out I had to learn and pick up how to do things, believe me I have had images that has been so sharp as to make your eyes water or images be super blurry and just look like sh.t.
After experimenting with many methods the following I have found to be the most effective.
Conver your image to 8bit and change color space to sRGB.
Then if take your intended display size (the size you want to display your image online), in this case we are going to use 1024pixels on the longest side and multiply it by 3, you can multiply by 1.67, but beware images tend to get oversharpened quite easily this way.
In this case: 1024 x 3 = 3072 pixels
Resize your image to your intended size x 3.
Create a duplicate layer (CTRL+J)
Go to Filter -> Sharpen and click on Sharpen (Do this 2 times on the same layer)
If you really want to finetune you can use Sharpen on the first duplicate, then duplicate it again and perform the Sharpen action on that layer.
Now resize your image to the finished size.
Here is the original image I started with just straight up resized to 1024 with no sharpening.
Here is the finished sharpened image.
And just for measurement here is an image half and half.
I also performed the sharpening multiplying 1024 by 1.67 (1710), but the mountains in the back started to show clear halos this way and the image generaly got too sharp.
And once you have done this a few time you can you can go to the Action meny and record an action for sharpening the image, I recommend making a couple for your most frequently used web sizes and one where you do not perform any resizing at the end so you can make custom sized images when needed.
If you have not performed any of these steps before it will have a dramatic impact on how your images are displayed online.
My output sizes
1024px - 1024x3 = 3072 - I use this on Google+, Flickr, 500px and DeviantArt.
960px - 960x3 = 2880 - Exclusively on Facebook.