Guide:Gnu IMP
From Game Making Tools Wiki
Tiling
If you have an image you want to repeat (tile), you can use a filter to do it for you. Find it in the menubar:
Filters → Map → Tile…
The window that opens is similar to the one for resizing. You can select different measurement units (pixels, percent, …), lock the width and height together so that change the same amount.
You can also check 'Create new image' to keep your original file intact.
