Hi Anna (and Simone)
I don't think NX-D will help you as it is mainly for processing RAW files (these have a file extension .NEF). Your camera seems to be producing JPG images. You are best off using GIMP. Here are the steps once you have downloaded the image to your computer and opened it in GIMP:
1. First, it's best to maximise GIMP so you are using the full screen on your computer. Do this the same way you would maximise any program window using the buttons on the top right of the program window.
2. Make any lighting adjustments you need. There are three options
(a) the simple option: go to the 'Colors' menu and select brightness-contrast. Move the brightness slider if the image is too dark or light overall. You may need to adjust the contrast slider. Use both of these in a way that keeps the image looking as natural as possible.
(b) the more complex (but a bit more capable) option: go to the 'Colors' menu and select 'Levels'. There is an 'input levels' graph. This is the only one you should play with. Under it are three arrows that can be moved along the bottom of the graph. The left is the black point the middle one is the grey point and the right one is the white point. Move the black point to the right to make dark areas blacker. Move the white point left to make light areas darker and move the grey point left or right to make the midtones lighter or darker.
(c) the most powerful option: go to the 'Colors' menu and select 'Curves'. This one is a bit too complex to explain her but I'd encourage you to look at the online help or better still find some online tuitorials on how to use this feature.
Remember, you always want the final result to look natural.
3. Once you have tidied up the lighting, resize the image for uploading to BLP: Go to the 'Image' menu and select 'Scale image'. Set the width to a maximum of 1400 or a height of 1050 (will depend on the image). Make sure the two are locked together (little chain on the right of these boxes) so the image doesn't distort when scaled.
4. Before uploading, the image must be sharpened as downsizing affects this. First go the the 'View' menu and select 'Zoom' setting this to 1:1 (100%). Next, go to the 'Filters' menu, select 'Enhance' then 'Sharpen'. Adjust the slider to sharpen the image just enough to bring back the detail. Don't over-do it. Note, sharpening should not be used if the image is out of focus to start with. If it is blurring, think about whether it is really worth keeping or uploading. Unless it's a rare bird or unsual behaviour, you are probably better off trying to take a better photo another time.
Note that I'll be away later this week and may not be able to respond to further questions quickly.
Cheers
Glenn