It is not an easy task to make the background of the Windows native edit control we use for editing transparent. You can search the Internet and see that many developers provide solutions, but it seems every solution is designated for a specific situation and/or has some caveats.
In iGrid, the edit box uses the background color of the cell (or the whole grid). Maybe, the best and simplest workaround for this task is to set the cell/grid backcolor to a color closest to the used image.
Another possible solution is to use an external control which supports transparency to edit grid cells. If anybody knows such a control, share the link here with the community.