This is really off-topic for this topic, but...
Indeed... Turns out that even though it only handles formats like TGA and PBM, it isn't even very accomodating with those! I couldn't resist playing some more, and I eventually tamed it a bit.
The insight came when I realized that the TGA file that worked had actually been created on the BeBox nearly 20 years ago! All the new ones I tried -- created with ArtPaint, or using ImageMagick 'convert' to transform a PNG into TGA -- gave garbage SVGs. And what all the latter have in common is that they have an alpha channel! When i finally used convert on a (high quality) JPG, that also worked.
So the solution, if the original has alpha, is to use, e.g.:
"convert logo.png -alpha off logo.tga"
Then autotrace can be quite good at generating an SVG. In fact I was a bit astonished to find that a fairly 'realistic' portrait turned into a handsome HVIF -- if one is willing to live with an ~11K icon!