How do I make the images on my blog larger

I have tried making my images larger on my blog. No matter what I try; using blocks that are full size, or Venis slider size, the images are still the same size.

This is a blog that I like, and I would like to be able to get my images the same size:
https://journal.thebeachpeople.com.au/journal/2018/6/25/joshua-tree-travel-guide

Any help would be much appreciated.