Closes #888 Fatal error when having image size with integer name

[Khadreal]

Description

Fixes #888

Documentation

User documentation

Should work as expected without any impact

Technical documentation

Type cast integer image size name to string

Type of change

• [x] Bug fix (non-breaking change which fixes an issue).

New dependencies

List any new dependencies that are required for this change.

Risks

List possible performance & security issues or risks, and explain how they have been mitigated.

Checklists

Feature validation

• [x] I validated all the Acceptance Criteria. If possible, provide sreenshots or videos.
• [x] I triggered all changed lines of code at least once without new errors/warnings/notices.
• [ ] I implemented built-in tests to cover the new/changed code.

Code style

• [x] I did not introduce unecessary complexity.

[wordpressfan]

@Khadreal @Mai-Saad @piotrbak I believe we need to merge this one, correct?

[piotrbak]

@wordpressfan Yes