Oh man, pictures are annoying to call in script.
I have a plugin that is NOT yet available for release (and my never be) that does this and a bunch of other stuff with pictures.
Here's the call to show a picture, with the arguments named
$gameScreen.showPicture(picturedId, name, origin, x, y, scaleX, scaleY, opacity, blendMode)
Here's the call to move a picture, with the arguments named
$gameScreen.movePIcture(pictureId, origin, x, y, scaleX, scaleY, opacity, blendMode, duration)
If you want to use percentages, then you should have variables, set aside for an x and a y coordinate.
Each time you show or move the picture, replace the coordinates with the variables that hold x and y.
You can easily increment the variables by percentage, a number of ways.
The thing is, calling game variables in code is long, too.
Here's what a completed call may look like:
$gameScreen.showPicture("Sword", 0, $gameVariables.value(1), $gameVariables.value(2), 100, 100, 255, 0)
It's a pain to type, and the more you type the more you risk mistakes, and then headaches.
But there, the best way to honestly do it is to have a place to keep track of the x and y, and use those as the arguments when you show or move the picture.
This will allow you to do any kind of math you want very easily, including working with percentages without inline math