JJ is correct. If you can upload to the site, then you can see what files are used in that site. The same program can be used to download the images if they are not already on your computer.
I recommend in the future, keep all files for your website on your computer in a dedicated folder. If necessary compress that folder for archiving. Especially if it takes up a lot of room. If your site is small then don't worry about compressing.
If someone else is maintaining your site, you can have them email you the image.
Simply due to the fact, that the original file can be larger than what is displayed in a browser. You always want to edit from the largest image file as possible.
hi first you right click on image and go to inspect element there you can find image link. Then you can see source path and also can download image to edit in photoshop. Later after edited you replace the image in backend to same folder that's it. If you website got any plugin to disable right click like in our website bodyguards, then you have to disable it.