Select from  Menu - File - Open Source Image
or use this Quick Button:
Open source image.

To modify already opened source image select
from Menu - View - Show Source Image

This screen will allow you to choose the main theme image and define its area that you want to use for the mosaic.

Source image setup screen.

Set how much columns and rows you need in the mosaic. You will see active area as a rectangle with cells over the main image. When active area is selected you may move and resize it using the mouse or keys with arrows.

Press Arrow key to move active area one pixel e.g. press Left Arrow to move active area one pixel left.
Press Ctrl+Arrow key to move active area 10 pixel e.g. press Ctrl+Left Arrow to move active area 10 pixel left.
Press Shift+Arrow key to resize active area proportionally e.g. press Shift+Left Arrow to decrease active area, press Shift+Right Arrow to enlarge active area.
Press Ctrl+Shift+Arrow key to move selection on its width or height. It is possible only if there are enough free space around selection.

To expand selection over maximum allowed area press Max button.<

If there are too many cells defined, it could be hard to see the image itself below grid lines. In this case uncheck Grid checkbox. Only outer bounds of selection will be visible.

To make project from a currently opened image press Apply button.