Thanks for putting this together! I see lots of broken directories in basically all TMX and TSX files when opened in Tiled (and similar issues in Unity). Looks like some files have been moved to _extras or packaged folders.
Viewing post in Pixel Art Unity Package - Mana Seed Tileset Collection comments
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.