The Layers dialog File menu includes commands for saving and loading the painted bitmap.
Opens a file dialog for choosing a bitmap file to load into the active layer, replacing its current contents. If you choose a PSD file, the PSD Input Options dialog opens with the option to collapse the layers or load a single layer.
Replaces the current layer contents with the contents of the Windows clipboard. The clipboard contents must be a bitmap.
For example, you could open an image file in Windows Paint, select part of it, press Ctrl+C to copy the selection to the clipboard, and then paste it into the layer using this command.