SmillaEnlarger supports Drag&Drop: You can drag a file or image from any folder or from another program offering Drag&Drop and drop it onto the enlarger window. The enlarger then tries to open it as new source.
If you drop a folder or multiple images at once, automatic batch processing is started instead of displaying a new source.
You also can paste the contents of the clipboard, i.e. use Copy within another program on a file or image and then in SmillaEnlarger call Paste from the File Menu ( or press Ctrl V / Cmd V ) to load it into the enlarger.
( This might not work in all cases. SmillaEnlarger tries to cope with things you throw at it, but will probably not always succeed. )