I digitized some terrific analog slides, but unwisely at times used an incandescent light bulb as my source of illumination and thereby left a very ugly black blotch in the middle of the digitized image.
Conventional photo editing software, like that attached to Picasa, don't work out for me. I don't have the artist's touch to use it.
So, I am looking for software which will allow me to "paint" over an ugly spot as a professional photographer could. Has anyone else had and solved this problem?
Just think of the television commercial for Microsoft Surface Book which shows a user "painting over" an ugly spot.
I see from your profile page that you use a microscope extender to do your digitising.
Since you have presumably successfully digitised other slides using a different light source, is it not possible to re-digitise the problem images avoiding the use of the incandescent light?
If you have an extensive collection of slides it might be worth purchasing a proper slide/negative scanner or a scanner with slide/negative attachments.
You might even be able to outsource the job to a commercial scanning service.
... I don't have the artist's touch to use it.
Unless the area to be retouched is an empty monotone region, I doubt that any software can "paint" over it satisfactorily.
Attached is a GIMP modified version of one of your slides where some dirt matks have been removed with the GIMP paint tool "Heal". If this is the kind of modification that appeals to you, then GIMP is an excellent way to go.