You have to save the file first.
PHP Code:
//#CLIENTSIDE
function onCreated() {
temp.img = "your base 64 string";
temp.img = base64decode(temp.img);
temp.filename = "temp_" @ int(timevar2) @ ".png";
temp.img.savestring(temp.filename, 0);
with (findimg(200)) {
x = y = 100;
image = temp.filename;
layer = 4;
}
}