Your code works fine for me. Make sure your FlowLayoutPanel is either Anchored or Docked properly, you have AutoSize set to True or your layout panel is large enough to fit all your picture boxes onto it or you will not see them all.
Here are some examples of some things you can try.
flowLayoutPanel1.Anchor =
AnchorStyles.Bottom | AnchorStyles.Top |
AnchorStyles.Left | AnchorStyles.Right;
or
flowLayoutPanel1.Dock = DockStyle.Fill;
or
flowLayoutPanel1.AutoSize = true;