[Relaunch] Missing images in success stories

[beta-ziliani]

The last two posts are missing an image. Alternatively we can select a better default image. Right now it shows the crystal logo trimmed in the top and bottom borders.

[straight-shoota]

I think this is okay. The posts could use an editorial review, now that they're more visible.

We should get a banner image which could be used as default image as well (https://github.com/crystal-lang/crystal-internals/issues/149).

[straight-shoota]

Working on this, but it's not quite there yet: https://github.com/crystal-lang/crystal-internals/issues/149#issuecomment-2022861397