There are probably a few steps you need to go through when returning home from a day of picture taking.
The real no-hopers can be deleted straight away, but the rest should be downloaded and at least given a second chance.
So this post suggests a few basic techniques that may be applied to any potential "keepers".
As your photos may be irreplaceable, its a good idea to store them initially in some logical place and work on copies, not the originals.
Folder storage could be like this:-
Pictures > 2016 > BandsHatchMarch > originals
...then maybe a second folder:-
Pictures > 2016 > BandsHatchMarch > edits
Cropping the image
For any picture you think looks promising, open the Gimp, save a copy in your edits folder, and use the rectangular selection tool to determine if it would benefit from cropping.
In this shot, I think the foreground tyres and the background crowd are distractions, and I'd like the car to be off-centre in the frame.
The next step is to see whether stretching the intensity range using Levels can improve the picture.
The Levels dialog initially presents you with a histogram displaying the intensity value and distribution of pixels in your image. Values to the left of the histogram are black or very dark. Values to the right are white or very bright. Since there is only a limited permitted range, it is normally a good idea to stretch the histogram out across the full range of possible values.
Use menu Colours > Levels... > Auto
|using the Auto feature in Levels seems to have worked for this image|
More often than not, your picture will look punchier just using Auto in Levels, but it doesn't always lead to an improvement. If you are not happy with the results, cancel or undo the change.
Then open the Levels dialog again and change the Histogram from Linear to Logarithmic. If there is a gap between the left edge of the histogram and the scale, slide the black marker to the right to meet the edge of the histogram, like this.
If there is a gap on the right of the histogram, close the gap by moving the white marker to the left. You can also experiment with the central marker to make the image appear lighter of darker.
Repeat this process for each of the colour Channels, and then OK changes if you are happy with the result.
sharpen the image
The car in this image does not look as sharp as I would like, so go to menu Filters > Enhance > Unsharp Mask...
Use the crossed arrows to view a magnified portion of the image containing some detail.
Increase the Amount is small steps (note: clicking on the slide track, not the sider button, increases/decreases the amount in 0.1 steps).
Compare the results by enabling/disabling Preview.
Do not over-do the Amount setting, as it generally gives best result for me between 0.5 - 1.2 in most cases.
reduce file size?
You can normally reduce the image size (and therefore the file size) if you only want to view your photo on a screen or maybe upload to a website. So zoom to show a good size on your screen and then note the % figure at the bottom of the window.
|As this is big enough at 30%, that is the value I rescale it to|
Once happy with this size, go to menu Image > Scale Image...
Set the Image Size units to %, make sure the chain is not broken, and change the Width from 100% to 30% (or your choice). The Height should automatically change from 100% to 30%.