Images live in src/assets/images/; the ones imported from Ghost sit in src/assets/images/og-legacy/.
For posts in src/data/blog/ (root level)
Most posts use a relative path with two ../:

Why two ../?
src/data/blog/your-post.md
↑ ↑
.. .. → src/assets/images/...
- First
..goes fromblog/todata/ - Second
..goes fromdata/tosrc/ - Then into
assets/images/...
For posts in a subfolder
A post in a subfolder, such as src/data/blog/dev_workflows/, needs one more ../ per level. The @/ alias for src/ works at any depth:

Common mistake
A single ../ stops at src/data/, which has no assets folder. The build then fails with ImageNotFound and names the path, so just check --only build catches it.