First, you need image editing software (Paint is not an option, it does not support transparency).
GIMP isn't the easiest to use, but is free and is quite powerful.
Then you can make your own textures (any size will work) which you will put in the "all" folder of the "textures" folder. You can also edit the default ones, but make sure to save them in the textures/all directory.
Then compress it and upload it to a site like
Dropbox or
omploader.