[sciurius]

That's what I expected... And that's why I went for a uri to designate the image source. When the uri points to an image somewhere on the web, it cannot be backup up and restored anyway.
So, in short, it is the users responsibility to make sure that the file is there where the renderer expects it to be.
Simple file names (no path) can be taken relative to the ChordPro file. This is not unlike automatic processing of audio files, and PDF/CSV pairs.
For a start, it's okay to only deal with simple file names and forget about the uri idea.

As for the values/ranges for each proposed parameter:

• parameter values are part of the parameter, separated by a =. No blanks.
  
• width: an integral number (pixels) >= 1, defaults to the actual width of the image. If it is too wide to fit, the image is scaled (keeping aspect ratio) to fit.
  
• height: similar to width. If both height and width are specified, aspect ratio may change.
  
• scale: scaling factor (fixed point decimal). If the resultant dimensions exceed the screen, scale to fit. If the height does not fit on the current page, make a new page. Default is to scale to fit, unless scale=0.
  
• center: if present the image will be centered on the screen, provided the actual width is smaller than the width of the screen.
  
• border: provide a border around the image. Default is no border (border=0). Just border without width is a 1-pixel border.
  

Does that make sense?
Note that screen width and height are subject to margins the MSPro may enforce.
Fallback: If an image cannot be found, act as if the {image...} was {comment: image...}.