What am I supposed to see on tensorboard images tab?
This is what I see using data from Pascal VOC that was converted using this example script: 
I am using Tensorflow 2. I think it's just the images preprocessed using the data_augmentation_options that were specified in your pipeline.config file.
I seem to recall seeing bounding boxes on the images in Tensorboard back when I was using the Object Detection API with Tensorflow 1. But I haven't seen any bounding boxes on the images using the API with Tensorflow 2. I have verified that the models subsequently worked. I just didn't see bounding boxes in Tensorboard. Note that I am just running training and then doing evaluation separately. If you specified eval_config or eval_input_reader in your pipeline.config, you might get different results.