I have old photo images scanned on Epson Photo Flatbed scanner
Were these photo images scanned from slides or negatives or prints?
If these were 35mm slides, it is difficult to get good, sharp, high resolution scans from a flatbed scanner. As mentioned by
@koraks, if the scans lack resolution or sharpness, there is not much that can be done about that.
What are the pixel dimensions of your scans?
I need to enhance/retrofit their quality and to be used for creating Digital slideshow
How do you expect this slideshow to be viewed? If the scans do not have much pixel resolution, then the slideshow may look OK on a small phone screen, but maybe not so good on a big screen TV or a computer monitor, especially if viewed from a close distance.
The word "retrofit" does not really work here. The word "enhance" makes more sense to me.
I have both Darktable and GIMP running on Ubuntu 24.04 PC here. Please advise which of them shall I use? I'm searching for an easy and not too complicated solution.
If all you want to do is adjust the contrast and the colors, any editing software should be capable of doing what you want. But you will have to learn how to work the software. Be sure to MAKE COPIES and do not attempt to adjust your originals, While you are learning, it is easy to ruin a photo, and you want to be able to start over with another
copy of the original.
So pick one, and learn how to use it. You can search for online tutorials, or you can learn by trial and error. It takes a while to learn, so have patience. For me, adjusting scans of slides did not start to get "easy" until after I had spent many hours learning the process.