Guide:Gnu IMP
From Game Making Tools Wiki
You can drag to rearrange the file tabs, but you have to grab them by a part that's not the thumbnail or the close button, very fiddly! If you grab the thumbnail it looks like you can drag it, but it doesn't nothing, ugh!!
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.
