from looking at your source code, i believe the issue may stem from using absolute positioning on your main div :] while it does appear on the browser screen to be centered, the preview images neocities generates are a different ratio, which makes the main div appear to skew to the right!

if you want to leave the layout how it is, you could create a custom thumbnail using this guide: https://control-neocities-thumbnail.neocities.org/ otherwise, to be honest, you could probably get a very similar centering effect to what you're looking for by using flexboxes or grids ^^ hope this is helpful!