When you run run_model.lua with -output_vis 1 (default) it will write the images and a json struct to this folder's data/ directory. These can then be viewed with this nice html interface.
For example, to evaluate a checkpoint on some VG test data:
th run_model.lua -checkpoint data/checkpoint.t7 -input_split test -vg_img_root_dir /path/to/visual-genome/images -max_images 10
and then start a webbrowser, e.g. python -m SimpleHTTPServer and open the view_results.html file!