Because your files will inevitably be in different folders than my files, you will need to reassociate those file paths for the sample maps to work correctly.
Double-click on each of those missing pngs and a dialog box will open so you can tell Tiled where your copy of that file is on your computer.
This is just an unavoidable part of transferring TMX/TSX files between computers, but you'll only have to update each associated file once.