Recent versions of fltk can drag/drop a variety of formats (e.g. PNG) not just text. But you may need to compile with the later ABI to get that (which is not the default for 1.3)
I don't remember what ABI that was added at though - might only be 1.4.?
That said, you still don't get much visual feedback, so to get that you possibly need to do that yourself.
In essence you need a small bordeless window to show the payload in, and make that track the mouse motion during the drag operation.
When the drag stops (i.e. the button is released) you hide that window and enact the drop target action in the usual way.
It's a bit of a hassle, but...
TBH, I don't much care for that effect anyway (I guess I'm too old!) but it does seem popular in a lot of apps now.
--
Ian
From my Fairphone FP3