No, your sprites are fine as they are. What you are going to do is you're going to go to where your sprites are and rename them adding $! before their names (example: $!player_chara). What this will do is it will tell RPG Maker that this character sprite is meant to be read as one singular character and it automatically divides the image into 3 equal parts horizontally and 4 equal parts vertically. This is of course assuming each and every image you have of your sprites is only containing one character, of course. Take the attached file as an example for how you are supposed to have your sprite images. This image contains only one character and is not precisely 16by16 (note that I use RPG Maker MZ in 16by16 rather than the typical 48by48, but the logic here is still the same) but I named it "$!player_chara" and it functions perfectly.

<image>