I think Strings are definitely the way to go. Pixel data cannot be read back without SSH's very hackish ReadBMP Module which is pretty slow. And as you yourself noted in the other thread, declaring a large array will bloat the file size and take up memory space even when it's empty.
If it's for a public module, I don't recommend using list boxes. The chance of conflicts with existing GUIs is large, and as you say, there are limits for the number of GUI objects.
If it's for a public module, I don't recommend using list boxes. The chance of conflicts with existing GUIs is large, and as you say, there are limits for the number of GUI objects.