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 to the height or width you select. For it to take effect (visually), you do have to resize the window. If you turn off Maintain Aspect Ratio, the image will be distorted to fit whatever window size you choose.
Right clicking a window will give you a menu of options for the window/image. The Color option allows you to select the background color (behind the image).
You can control whether the tool window stays on top of everything or not on the Window menu or the toolbar. You can also hide the main window when you are ready to copy the screen.
In many cases it is easier to use keyboard hotkeys. These are shown on the menus and in the hints for the toolbar buttons. Pressing the Alt key or F10 will bring up the main menu of the tool window. The simplest way to capture the collage is to hit F12 to hide the tool window, then the Print Screen key. Press Ctrl-N to create a new image window and Ctrl-V to paste the collage. Then save it, of course.