Reformat picture on main page

### Summary

Currently the main picture has two straight corners and two curved corners. This bothered multiple people so probably worth changing.

### What needs to be done?

- [ ] Make the corners of images consistent - either all round or all square?

No file, test, or entry point is identified in the issue. Locate the main page image and inspect its current corner styling first. Done means the image corners use one consistent style, either all round or all square, with the chosen result reflected on the main page.

Written by the indexing model from the issue text.