CSS: Part layout & default image placeholder (for records with no part images)

[jamieu]

Previously we surpassed the (default/blank) image placeholder if none of the records had an image.
https://collection.sciencemuseumgroup.org.uk/objects/co205092?534

I also had a vague memory that we had dealt with the cases where we only had two part records and reduced their size, but maybe I am mistaken and the solution only dealt with the specific case of one part image. Two part images at this size feels quite overwhelming on a desktop browser.