Specify sizes for images

[php-coder]

All series images on the page /series/{id} should have height and width attributes specified. Because these images are image previews that were generated by the server, we should find out a way of fetching info about image dimension during image preview generation.

See also: https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Tips_for_authoring_fast-loading_HTML_pages#Specify_sizes_for_images_and_tables

[php-coder]

Blocked by #387