$obj->method("foo", "bar") gets translated (more or less) to SomePkg::method($obj, "foo", "bar") -- the object is passed into method as $_[0]. Your code for makeImage should expect the object as the first argument if you plan to use it as a method.

If the function works as you desire right now, you probably don't need it to be OO, as it doesn't get passed the object anyway.