Just can't seem to get a png file in 24-bit for CAW

I'm still playing TS3 and, sometimes getting bored, I edit worlds I play. I've got a small world I'm pretty much done tweaking but no thumbnail. I did download a 256x256 thumbnail initially (because you have to) and it wouldn't load up because of the 24-bit issue. I know it took several tries to finally get it in 24-bit, but I have NO IDEA what I did. 

Just because it will be asked, yes, I have the screenshot. Yes, I've cropped and resized to 256x256. Yes, I've chosen .png. I've attempted this in Paint & Paint 3D. I've been into Photoshop which, frankly, I've had for more than a decade and never really figured out how to use and could never find any way to crop by pixel. I did try to save the thumbnail I'd cropped elsewhere through PS but that didn't "fix" anything. I don't wish to download some sort of "trial" program in order to get this single item to work, especially since I know it can be done, just no idea how. I've watched a YouTube video that relied on a trial program so that's no use. I've done searches but they haven't helped, they are few in number and they are years and years old. I've actually done this stupid thing a few times over the course of the past 10+ years, even though it's "obviously" been a while, but can fully recall that it was always something very difficult and seemingly hit or miss! 

It would be so much appreciated if someone runs across this question and can give me a quick and dirty bulletpoint on what will save my .png file in 24-bit mode.