JPEG Collager is a JPEG (and BMP) viewer which shows each image in a window without any border. The best way to see how it works is to play with it a bit, but here is a summary of use.
You can open any number of images. Each one is manipulated individually. On the tool window in the title bar you can see which image the mouse is currently over. The active window (the one which will be affected by operations) is indicated on the left of the status bar, with its properties shown on the right. When you see the icon become a hand, left dragging moves the window and right dragging moves the image within the window (unless it is stretched to fill the window). The window can be sized by dragging the edges.
The Stretch option causes the image to fit the window. By default, Maintain Aspect Ratio is on. This means that the picture will not be distorted but resized...