According to pcgamingwiki: https://pcgamingwiki.com/wiki/Final_Fantasy_VI

You can use Borderless Gaming, Gedosato, Special K and I believe SweetFX and ReShade can force a game to run Borderless.

Not as easy as you expected but should not give you too much trouble!

Based on the post by ZephyrAm and this wallpaper:

https://i.kinja-img.com/gawker-media/image/upload/b06m1i51beklgkpf4upj.jpg

I decided to make my own character protraits since I love some of the characters in that image.

I changed the protrait of every party member and found a Kefka and an Esper Terra very similar to the art from the wallpaper.

Here are the Large Portraits:

Terra: https://www.dropbox.com/s/3eri8btcfdsi9hj/F_00000.png?dl=0
Mog: https://www.dropbox.com/s/yy8auapze3k78ym/F_0a000.png?dl=0
Gau: https://www.dropbox.com/s/lc0oo8kbqzwbton/F_0b000.png?dl=0
Gogo: https://www.dropbox.com/s/vrlxkii31kaf3s9/F_0c000.png?dl=0
Umaro: https://www.dropbox.com/s/wxjya9c5rkkpy6x/F_0d000.png?dl=0
Locke: https://www.dropbox.com/s/6371842yb3kbp5m/F_01000.png?dl=0
Cyan: https://www.dropbox.com/s/xtgqwjp2y8xhgoz/F_02000.png?dl=0
Shadow: https://www.dropbox.com/s/ufx426otdyzz4fh/F_03000.png?dl=0
Edgar: https://www.dropbox.com/s/tt6ne4vrcnd8d9c/F_04000.png?dl=0
Sabin: https://www.dropbox.com/s/wfriwisrw01t39j/F_05000.png?dl=0
Celes: https://www.dropbox.com/s/67ldmy89kyhp6fu/F_06000.png?dl=0
Celes Green: https://www.dropbox.com/s/czpxdk6iplinpjh/CelesGreen_L.png?dl=0
Strago: https://www.dropbox.com/s/asg4dyol7wccbjy/F_07000.png?dl=0
Relm: https://www.dropbox.com/s/i7gjj2io1mk9ff5/F_08000.png?dl=0
Setzer: https://www.dropbox.com/s/9v67n4wth04hcv4/F_09000.png?dl=0
Esper Terra: https://www.dropbox.com/s/aa6m7drn072omsg/F_10000.png?dl=0
Kefka: https://www.dropbox.com/s/9ecu1yp9tbw88mt/F_14000.png?dl=0

Here are all the Large Portraits with 2 versions:

With Purple Celes: https://www.dropbox.com/s/0gobo9pg6rhpxby/PortraitsPurple.png?dl=0
With Green Celes: https://www.dropbox.com/s/cfgbqso04ibvws2/PortraitsGreen.png?dl=0

The Medium Portraits:

https://www.dropbox.com/s/73mn7onazn9gp9h/Portraits_M.png?dl=0

The Small Protraits:

https://www.dropbox.com/s/4lrki5l1xymeucs/Portraits_S.png?dl=0


Here are the executables for every protrait:

Purple Celes: https://www.dropbox.com/s/ambg68l8wc78xtf/Protraits.exe?dl=0

Green Celes: https://www.dropbox.com/s/mjxx0heeiqi1fch/CelesGreen.exe?dl=0

Hope you like them!