That is a compression setting in Irfanview I reckon? Compression is a way to make an image less 'heavy' in file size; the number of kilobytes.
Compression does not create those jagged edges we see on your aircraft, lack of AA (Anti Aliasing) does.
Do you have a video card in your computer and if so, which brand and model is it? Or is it integrated in your motherboard?
If you have a video card in there you might want to check if you have AA enabled.
You can check if you have AA enabled by right clicking on your desktop and select PROPERTIES.
In the window that appears click the SETTINGS tab and click on the ADVANCED button in the lower right corner.
You should now see a couple of tabs, generally those are: GENERAL - ADAPTER - MONITOR - TROUBLESHOOT - COLOR MANAGEMENT and either something like NVIDIA or ATI.
Click on the last one and you should be able to see something like PERFORMANCE SETTINGS or something similar to that.
See if you have an option for Anti Aliasing and what value it is set to ...
Let us know if and how this works out
