fix: set img role to thumbnail container

[gormus]

looks like I missed an essential part last time discourse#77

According to MDN:

Any set of content that should be consumed as a single image (which could include images, video, audio, code snippets, emojis, or other content) can be identified using role="img".

You shouldn't count on the alt text of individual images for conveying context to assistive technologies; most screen readers will consider the element with role="img" set on it to be like a black box, and not access the individual elements inside it. Therefore, provide a comprehensive overall descriptive alt text for image, either in the surrounding text, or by using an aria-label attribute, with alt attributes for search engines or sighted users to be written to the page should an image fail.

source: https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Reference/Roles/img_role