There are two ways to save your photograph as a JPEG file in Photoshop, and both have their strengths and reasons for choosing one over the other. The first we'll be looking at is Save For Web.
Under Photoshop File Menu, choose Save For Web, and select the 4-Up tab in the upper right of the window (Image 3), which gives you four views of the photo: the original, plus three previews of image quality at various compressions. To the right of the previews, select the drop-down for JPEG in the top left output options dropdown (GIF is the default setting). You will have to highlight each of the previews and select JPEG or another file type for each preview (for our purposes, just make them all JPEGs). Once you have selected the JPEG option, you will see a slider titled Quality (circled.) Slide it to 59 and the top right preview will show a file size of 98.41K! That's well under the file size limit. Click Save and you're done!
The two lower images show the file size if you choose 25 quality and 0 (zero) quality. Try this on your computer and you'll better be able to see the relationship between image quality and file size, as the lower right preview, set at 0, will be really 'fraggy'. For websites, this is handy, because you can select your exact level of acceptable image quality while keeping total page load time quick.
It is important to note that Save For Web strips EXIF information (data about the image capture ISO, shutter speed, etc.) and any user-inputted File Info, such as captions or copyright info, to make a smaller file size. If you need to send along embedded captions or wish to preserve the EXIF data, choose Save As JPEG from the Photoshop File Menu.
Before saving as a JPEG or choosing Save For Web, make sure your image is flattened, with no Alpha Channels otherwise you will be warned that you can only save a copy. Select Format JPEG, name your file, and click Save. A new window will pop up (Image 4) with your JPEG saving options. Check the box for Preview (circled) and move the slider to a desired quality. We chose 5 on the slider, resulting in a saved file size of 93.64K. Once again, we've turned our 558.1K screen-sized image into a saved JPEG that is under the 100K file size limit! And if we want to see what the tech specs are, we can still view the EXIF info.
It's pretty simple, once you understand it. Any time you see a file size limit, be it 100K, 500K, or 1MB or whatever, follow these steps and you'll be on your way!