Qiewt is probably the tiniest, simplest image viewer out there. It displays any image thrown at it (i.e. dragged and dropped on it) on your desktop - just the picture, no borders, no frame, no toolbar.
Move it around with your mouse or keyboard. Scale it to different sizes. Read on for a complete feature list.
Drag and drop any number of image files on Qiewt to display them
Move the images around on your desktop by dragging them with your mouse or pixel-wise by means of your keyboard's arrow keys
Scale the displayed image using your mouse wheel or the + and - keys
Scale or move in bigger steps by holding the Ctrl key pressed
Open images in new windows by pressing the Ctrl key while dropping the files on an existing Qiewt window
Reset to original image size by pressing the middle mouse button or Esc
Close Qiewt with Backspace
Click with right mouse button to find out original and displayed size
You can pass any number of image files on the command line
Now... what's it for?
Why, you may ask, do I need something that simple? Life is more complex than that!
Well, I don't know what uses you may come up with, but I do web design and I often need to have a quick look at whether some graphic fits into some existing web page or whether some image slices for page backgrounds fit well together. Without Qiewt I would have to take a screenshot of the webpage, open it in a graphics program, add the other graphic, etc.
Qiewt is written in Python using PyQt4. So everything you need is a recent version of Python, PyQt4 and Qt4 which are already included in every up-to-date Linux distribution.
To run Qiewt, just make sure the mentioned prerequisites are installed, download qiewt.py, make it executable, click on it (or something equivalent) and there you are!