Some basic guides.
You'll need to create and save the image in Photoshop first.
Then click Image menu / Image Size.
Begin at bottom & work your way up to top (really!)
- Bicubic Sharper (which can also be set in Preferences)
- Checkmark all 3: Resample Image; Constrain Proportions; Scale Style.
- Document Size: Resolution: 72ppi (96ppi doesn't make the image smaller, in this example at least.)
Then save File as a .jpg.
File / Save As / .jpg.
All of the measurements in inches or centimeters, the pixel height and width, and the resolution, stay the same during the conversion process.
In my experience, changing the ppi and the physical measurements in inches or centimeters, etc, is the way to resize images for the web. Changing the Quality measurements while saving as .jpg doesn't do it.
NOTE: As you reduce the size of the image, the sharpness of detail increases.