Well, I know this is a very old topic and that a lot of people already explained this but I just felt like making this guide, so if you are not interested in reading another MM7FC mod tutorial, just don't read this topic, ok? Good. Let's start.
First, you should open your MM7FC folder. You should have some folders with these names:
"bgm", "image", "pc", "se" and "stg".
"Config.exe", "config.ini", "pxtone.dll", "readme.txt" and "rm7fc.exe"
You should open the "image" folder and find there a lot of files with the termination ".bmp".
"What's that?" Well, it's a old but still working image format used for the old Windows computers.
But don't start editing it yet, because normal Windows 7-10 MSPaint or any else program like Paint.net and Gimp won't work.
Only XP Paint works for .bmp's.
I can't tell you where you can find the file for that, but I'm sure that by searching in your browser "windows xp mspaint download" you'll find a safe download.
Now just open the XP MSPaint exe and with that you can open and edit the .bmp files. (If you use all the other image reading/editing programs, it'll corrupt the file and it will look like a mess).
You should know that every file has a sort-of-grid programmed on it so that the game recognizes it. Megaman's .bmp has a 32-32 grid.
By this point you should be able to make anything. Just make a copy of the original and mess around with it. Trust me, it's fun!
I hope this was useful for you. Have a nice day! :D