Often photos of their original size are poorly suited for posting on the web or browsing on mobile devices.
Fortunately, to reduce their size with the popular editor Photoshop is very simple.
Open Photoshop and from it, through the "File" menu, and then "Open", open the photo that interests you.
Select the "Image" section at the top of the program window.
Go to the "Image Size" setting.
Specify the new dimensions of your photo in pixels. If necessary, if you want to change the aspect ratio of the picture, uncheck "save proportions".
Save the thumbnail.