Fix for optional parameters.

This is a proposed fix for the find_images_in_list function.

If you already have a fix, feel free to close this pull request.

This issue was reported here:

• https://wordpress.org/support/topic/cant-crop-new-thumb?replies=1#post-6563593

• https://wordpress.org/support/topic/yoast-xml-posts-sitemap-broken-by-nextcellent-gallery-19252?replies=2