Automatically check blog post image requirements

**Short description**: Automatically check (in GitHub action) main blog post images for their ratio and size.

**Location**: Blog

**Why have this**: We want blog images to have a certain ratio, so that the space in the blog overview is used efficiently, and of high enough quality to display nicely on social media.

**More info**: We can use ImageMagick's [identify](https://imagemagick.org/script/identify.php) through GitHub action. Ratio + minimum size to be determined.