I am not sure if this is a little known trick, but even if it's well known I am going to say it again. One of the features in Photoshop that can save you a ton of time and make your pictures ready for the web in a hurry is the "Save for Web" feature. It is now located in File > Export > Save for Web. I have it as a shortcut of Cmnd+Alt+Shift+S, so I can get to it quick.
This will be really helpful if you post pictures on the web most of the time, which is like 99% of us. A couple of cool things the dialog does is convert the image to 72 ppi, which is ideal for screens, and it allows you to convert the image to sRGB so that it will look correct on other peoples monitors. As a quick side note, make sure any image you post on the web is converted to sRGB, otherwise you could get some funky results. You can also resize the image and choose the quality of output so you can keep the file size down as low as possible.
A couple things I see happen all the time is the either the file being way to big, or the color space being wrong. You can do these things in Photoshop manually, but this allows you to do it all at once. Hopefully a few people find this useful...go be awesome! P.S. I this to the image below...and every image on my web page!
