On a 21:9 monitor, a lot of background images get stretched so a way to indicate the scaling method used would be very appreciated.

Perhaps something like:
@time-time: [scaling] image1 .. [scaling] image2
where scaling can be one of the options supported by wallutils (stretch, fill, scale, tile, center)?

For the purpose of sharing .stw wallpapers, it should be possible to package the .stw file and the associated images into a single directory. The wallpaper should work the same when referred to anywhere on the file system.

To accommodate for this, I propose specifying that relative paths in the .stw file are relative to the parent directory of the file.