If you really want to use xnb mods you can read more about them
here.
To merge xnb files you can use
xnbcli.
- Make a backup of original craftables.xnb (found in Stardew Valley\Content\TileSheets).
- Copy the craftables.xnb file to "packed" folder and press "unpack". In "unpacked" folder you'll get a png file that you can edit.
- Then using a graphics program like photoshop, gimp or other that you like, replace the vanilla scarecrows with the new ones.
(note: in craftables every object take space of 16 pixels width and 32 pixels height)
- After it's done save it as craftables.png and place it in "unpacked" folder. Then press "pack" and it will merge it to an xnb file (you can find it in "packed" folder).
- When you have your modified xnb replace the vanilla craftables in the game files (Stardew Valley\Content\TileSheets).
Again, it's not recommended, use at your own risk