That's simple. You need to crop your original to a maximum height of 728 pixels. If you upload anything larger than that, it becomes bigger than 768 pixels by the time the Airliners.Net banner is added at the bottom of the pic. And if this happens, it is resized down, with a subsequent reduction in quality. Specifically, you will find that the pic in the database is not as sharp.
You can check if this has happened to your shots by downloading the pic in the database, and then comparing the size with that of the original. In fact, if you use Internet Explorer, simply open the large version, right click it and select properties and it will tell you the size of the file in pixels.
I would suggest that if this happens to you, and you are not satisfied with the quality of the pic in the database, then you should do the following:
1. make a note of the pic number in the database
2. crop the pic on your hard disk to a size of 1024x728 pixels (or smaller).
3. go to "add photos"
4. click on "re-upload page"
5. enter the pic number you want to update, your email address, and browse to the cropped version on your hard disk
This has caught me out a few times. I always resize/crop to 1024x728 max. but on odd occasions I have forgotten to crop the vertical size of a pic, and have only discovered it much later by accident. It's most annoying, as there's no feedback from A.N to let you know a pic has been resized.
Hope this answers your question and helps you fix it.