In Photoshop you have the option 'Save for web' (at least you have this option from version 5.5 and up)
You have to make sure that the image is already in 72dpi, for higher resolution has no added value since it is the monitor resolution.
If you use JPG compression it is possible that when you use High comression you lose quality.
Another thing you should look out for is the image scaling. Do not use the image tags to rescale an image on the internet, that is a certain loss of quality.
Save the image in Photoshop using 'Save for Web' and make sure the image appears on your screen at 100%. (have preview selected, that way you can see how it should appear online, even if you change properties such as compression when using jpg)