If you need to quickly view an entire folder of images, Image Viewer from Abacus does the job. It's a one-trick pony for Mac OS X (10.7 and higher), but does its trick well. Plus, it's only US$2.99 at the Mac App Store.
With Image Viewer, you can open a file and then view all image files in that folder by clicking the mouse or pressing the keyboard. This isn't something I normally do very often, but your needs may differ.
If you use Image Viewer frequently, you can make it your default app for viewing pictures. The interface is simple. You can view images and navigate with keyboard arrow keys, toolbar buttons, or the main menu. You can do the same with thumbnails. For example, double clicking a thumbnail will also open the image in single view mode.
You can open images by selecting Open from the main menu, right clicking in the app, or on the toolbar. You can also drag a folder or file to the app to open all of the images in the folder or enclosing folder if you selected a file.
Image Viewer can open any folder that you select. But If you open a photo or image file outside of your pictures folder you have two choices to view all of the images in the same folder.
You can choose a folder, your home folder, or a disk drive to add read only permission. This location is saved, and you won't be asked for permission again inside that folder or drive. On the other hand, you can open the folder that holds the files and grant temporary permission to that folder.
On the downside, Image Viewer doesn't jibe very well with Photos. You can drag and drop pictures into the former from the latter. However, it would be nice if you could, for example, drag an album directly from Photos into Image Viewer.
It has room for improvement, but if you need to get a fast view of lots of images inside folders, Image Viewer is only going to set you back a buck.