Fix admin welcome illustration not found

What does this MR do?

This MR fixes an illustration not loading in the admin welcome page. Note this only happens in a production environment and can be recreated in development by following the steps in #34768 (comment 240588325).

Screenshots

20191105_bug_fix_welcome_images

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Related to #34768 (closed)