Используете ли вы конвейер активов? если да, сначала выполните прекомпиляцию с помощью
RAILS_ENV=production rails assets:precompile
, затем попробуйте
# attempt 1
<img class="thumbnail" src="<%= 'assets/' + design["thumbnailURL"] %>">
# because the images would be compiled and under public/assets/
# I don't know exactly what's design["thumbnailURL"] contain, so I list this attempt
# attempt 2
<%= image_tag design["thumbnailURL"], alt: '...', class: 'thumbnail' %>
# image_tag will also append SHA256 suffix in the filename
# in production environment, all the assets will be appended a SHA256 to let the browser know its the latest assets, ex: aaa.jpg will be aaa-908e25f4bf641868d8683022a5b62f54.jpg, it may be the cause the browser cannot find the right one and return 404
# attempt 3
# if you don't want the images being precomiled, just put original images under public/ rather than app/assets/images/
# design["thumbnailURL"] should be the relative path to public/
# and you can remain your <img ...> same in view
# however, if you update the image, the users who already have been to your site would cache the old image and your new image will not be shown on their browser
развернуть новый код и посмотреть, работает ли он