Fix podcast image size - iTunes Thumbnail Bug

[hermify]

Instead of taking thumbnail for podcast in itunes, it takes full image size. Fixing bug.

Types of changes:

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ x] I have read the CONTRIBUTING document.
• [ x] My code follows the code style of this project. (WPCS)
• [ x] My change requires a change to the documentation.

Brief description of the proposed change:

Any other info: