.var filenamelist = List(all filename entries)
...
conversion loop with another filenames from the list:
.var filename = filenamelist.get(loop index)
.var importbitmap = LoadBinary(filename + ".tga")
...
some code to convert data and fill array with it
...
.pc = picturedata filename(index) 
.fill 8 * rows * columns, bitmaparray.get(i)
.fill rows * columns, screenmaparray.get(i)

.const twisterColors = List().add(peptoBright.get($8), peptoBright.get($7))
.const numFrames = 7

.var twisterFramesFiles = List()
.for (var i = 1; i <= numFrames + 1; i++) {
    .const fileName = "assets/twister/sprites frame " + i + ".png"
    .const file = LoadPicture(fileName, twisterColors)
    .eval twisterFramesFiles.add(file)
}

.macro cutTwisterFrame(frame) {
    .const file = twisterFramesFiles.get(frame - 1)
    .for (var y = 0; y < layout.size(); y++) {
        .for (var x = 0; x < layout.get(y).size(); x++) {
            .eval twisterPointersList.add(toSpritePtr(*))
            cutHiresSprite(file, x * 24, y * 21 + 6 - x)
        }
    }
}

.pc = *
cutTwisterFrame(1)
.pc = *
cutTwisterFrame(2)
...