Mac or Windows? If Mac right click on the file, choose "Get Info" and then under "open With" enter your program of choice.
If Windows I forgot since I haven't used Windows since 1998
If on a Mac, you can quick look any JPEG, TIFF, PNG, and PSD by clicking on the image in the Finder then pressing the space bar. Same action works for many raw files as well (I've tried DNG and ORF).
If you press the up- or down-arrow keys, you can walk through the contents of a folder. If you select a group of files and then press the space bar, you can also click a grid button in the quick look window and see the group as thumbnails laid out in a grid.
You can open all the same files with Preview.app for viewing too...
There are many ways to "just look at the photos."
G