If you are looking for a simple and lightweight image viewer for your Android device, you might want to check out jp.garakuta.imageviewer by Garakuta Software. As the name suggests, it is a straightforward image viewing app that allows you to browse through your image files in the internal storage and SD card.

One of the best features of this app is its implementation of a secondary cache of images, which enables fast display of images. You can easily access your image folders, view thumbnails, and scale images to fit your screen. Additionally, the app allows you to delete files and even has a camera start-up shortcut for quick access to your camera. The thumbnail of the secondary cache is also a handy feature that allows you to view your cached images.

The app supports popular image formats such as JPG, PNG, BMP, and GIF, making it a versatile image viewer. The app also includes a small banner ad, but it does not interfere with the app's functionality. Overall, if you need a simple and efficient image viewer for your Android device, jp.garakuta.imageviewer is definitely worth a try.