Haw make a big image for characters

hy, im new here , and i woudlike to excuse my english, i dont talk spanish, im from france.
it made long long time i mod the game (something like 4 or 5 years) and for real i learn a lot with spanish community,  haw change characters/music/map/attack.. you know all this.
lot of things, but i never found before a forum to can talk about the amazing game.
im very glad to be here.  sorry for this little introduction.    

so My ask is about the BIG Pictures,i know haw to put thems in game, but wat  i woudlike to understand and learn is :

"haw create one"

to can personaliz the game more.
I love wat did peoples, but if we could do this, the game will be so much amazang^^      

because i see everywere tutorial "haw to put little pictures of selection"  
but i never found someone who can explain haw to createone, and convert in ".unk"

??????????????????????????????

it is so simple that nowane talk about ?
or maybe is just impossible?    
but haw did modders for goku ssjblue??  

sory for the long post.

i hope to have a anser,
cause i av also another question, but after...
First i need to know if is possible and if is very difficult

no anser