General: Organize images

[ngrayluna]

Description

More cleaning for the big migration.

Replaces #3176 after rebasing onto origin/main.

[github-actions]

Readability impact

Word-weighted Flesch-Kincaid grade change across 3 changed pages: +0.0 (unchanged).

Lower Flesch-Kincaid grade and higher reading ease both mean easier to read. This check is informational and never blocks a PR.

Human readability

Page	FK grade before	FK grade after	FK Δ	Reading ease Δ	Direction
models/app/features/custom-charts/walkthrough.mdx	9.4	9.4	+0.0	+0.0	unchanged
models/app/features/panels/parameter-importance.mdx	13.3	13.3	+0.0	+0.0	unchanged
models/app/features/panels/scatter-plot.mdx	7.3	7.3	+0.0	+0.0	unchanged

AI agent comprehension

Rated 0-3 (higher is easier for an agent to parse and act on).

Page	Before	After	Δ
models/app/features/custom-charts/walkthrough.mdx	2	2	+0
models/app/features/panels/scatter-plot.mdx	2	2	+0
models/app/features/panels/parameter-importance.mdx	2	2	+0

_{Curated-docs baseline median FK grade by type: conceptual 10.5, procedural 8.8, reference 9.4.}

From workflow run 33679546615

[dbrian57]

LGTM