Responsive Gallery Images

Recently I added a gallery to my website - I am usually not using galleries much. Initial problem was that the thumbnails were too small, so I increased the thumbnail size and recreated existing thumbnails with a plugin (there are several). At that time, when I started to build the theme I had the responsiveness removed for the side to be static.

In the meantime I changed my mind and re-enabled responsiveness, now leaving me with the problem that the thumbnails (that are inserted without an option I could change by selection - or am I missing something?) are obviously too small at certain screen sizes. To solve the problem the gallery images would need to have the img-responsive class applied and full-width images used instead of thumbnails.

What would be the best way to accomplish this with the greatest amount of compatibility (to existing standards)?

Would you be willing to just use a plugin all together for this feature?

Such as the Royal Slider? Would give you the exact options you are looking for as well as full range to easily edit the output of the slider images or the thumbnails?

The examples may seem very basic, but it starts that way and is extremely easy to customize.

Try Fotorama. Nice and free