Yes, that is keyboard intensive work, and boring as hell LOL. Huge time sink for sure.
As above - Paint is the easiest way to resize a cell pic or anything else. Example:
This is a 4.3m file, 5312 x 2988 pixels. In Windows, you can right click the file and Open with Paint. (usually LOL) Click Resize, select the Pixels button and pick a decent width - 800 is usually good, 1024 is another good number that will leave in a lot of detail and not blow the forum thread off the earth. Then click OK. Here, I used 800 x whatever - the program will keep the aspect ratio if the Aspect box is checked. Use Save As to save under a new name to preserve your original file.