Images in reports are too large

After merging #208, images in reports are too large.

To reproduce, try reporting a box plot.

I assume that WebKit treats all images as 72dpi. Perhaps adding `zoom: 0.5;` to css would fix it. **Edit:** yes, but only if the image was taken on high-dpi screen.

Reproduce the issue by reporting a box plot, then inspect the report image sizing and CSS related to the changes in #208. Compare behavior on standard- and high-DPI screens and confirm that report images render at the expected size without breaking either case.

Written by the indexing model from the issue text.