The correction that Scangear applies is not much different from what you can do with Photoshop or GIMP or another image editing tool. When scanning, please do yourself a favour and use a lossless format (TIFF) and don't scan to a JPG file, as you lose information straight away. Open the TIFF file in the editor of your choice, apply corrections and then use 'Save As...' to save a copy, thereby preserving the original scan. That way you can always go back and try again.
You can also create a copy of the TIFF file first, before starting the editing process.
When using scangear, you're basically taking the same steps but applying them all at once. It doesn't change a thing about the way your image is scanned, and you don't have the luxury of going back and trying again without re-scanning. The other question is whether scangear will preserve all the image data. Sometimes scanning software is less capable than a professional editing suite, and you may lose some detail through their automatic process, which you would not have lost when using different software.
What it boils down to:
- Use the scanning software you're comfortable with. Save to a lossless format and set the dpi high enough so you capture all the detail. For a photograph, anything over 600dpi is overkill as the resolution of the print you started with (assuming it's not printed at a very large size) won't show anything smaller.
- You can get into very complicated arguments on how to process the resulting image, but make sure you work on a copy, preserve the original scan.
- My advice would be to find an image editor that allows you to use an automatic tool (Auto Tone in photoshop, setting a white and black point in Lightroom, or using the automatic tool in that, etcetera) as that way you won't have to get into the details of how you work with color values and levels. The problem here is that each bit of software has a different way of describing the various tools, settings and parameters. Find one that works, watching tutorials on youtube is a good way to get an idea of how difficult something can get. See if you can find trial versions if possible.