At least you have two starting and three ending curved brackets on that snippet:
{
"Format": "1.29.0",
"Changes": [
{
"Action": "EditImage",
"Target": "Portraits/Sebastian",
"FromFile": "assets/Sebastian Portrait.png"
}, // there are two records; they should be separated with a comma after closing
{ // and the second one started with a new bracket
"Action": "EditImage",
"Target": "Characters/Sebastian",
"FromFile": "assets/Sebastian Sprite.png"
}
]
}
Edit: The good starting point is always to look for unbalanced brackets, and extra or missing commas. The "fun" really starts after all those are fixed and it still won't work...