One of the best shortcuts that my mac gives me is the ability to use QuickLook. This a beautiful shortcut that allows me to see an image or document without opening it up. What you do is select the file(s) that you wish to see and just press the spacebar.
You can then use the arrow keys or swipe to view each file. There is also a share menu in the top-right-corner where you can then send the image concerned.
You also have a fullscreen option in the top-left-hand-corner and a thumbnail view as well there. There is also a button to open the file up in the application concerned, however you can only use quick look on a few file types. Basically images, some video file formats and txt files. What about other file types?
Personally I need to preview more text files like markdown files, son files, more video file types, and also Blender files and Illustrator etc. What can we do? Well there are a whole host of free QuickLook extensions that you can download and install and use. have a look here (http://www.quicklookplugins.com/).
If you look in your Macintosh HD > Library > QuickLook folder you can see a list of installed extensions and this is where you can add new QuickLook extensions.
The zip, epub, markdown, json quicklook extensions I find invaluable. There is also a paid set called ArtView which allows you to preview Creative Cloud and Creative Suite documents (http://www.code-line.com/artview/).
In short have a look at your QuickLook folder - see what is installed and check what files you use regularly. There may be a QuickLook extension out there for you that makes your life much easier.