You can confirm the supported formats here: http://help.nchsoftware.com/help/en/pixillion/win/formats.html
The situation is with all the images try this,
Let's try doing a clean install and delete all the cache of the program. Uninstall your software first, then go to the folder location below on your computer then delete the Pixillion folders:
C:\ProgramData\NCH Software
C:\Users\(Your computer profile)\AppData\Roaming\NCH Software\Pixillion
If you can't find the AppData folder, set Windows to show hidden files and folders with the steps from this link: https://support.microsoft.com/en-us/help/14201/windows-show-hidden-files.
After deleting the Pixillion folder, restart your computer then install it again from the next link https://www.nchsoftware.com/imageconverter/pixpsetup.exe
That should do it. Please try it again after that and see if that fixes the issue.