Check images for dark mode compatibility

### What is the problem?

There is an image in the memory management section that has no background (transparent) and the text is black so can't be seen in dark mode: https://carpentries.github.io/instructor-training/05-memory.html#chunking

Maybe this can have a slightly grey background to make it work with both themes?

### Location of problem (optional)

https://carpentries.github.io/instructor-training/05-memory.html#chunking

Open the memory management section at https://carpentries.github.io/instructor-training/05-memory.html#chunking and inspect the image in the rendered page under both light and dark themes. Identify the image asset used there, adjust its presentation so the text remains readable in both themes, and verify the result in the lesson preview.

Written by the indexing model from the issue text.