The Shift+Cmd+4 key combo is common from all the way back to Mac OS 9 (and maybe earlier?) to all Mac OS X and current MacOS versions and takes a screenshot of a selected area on the screen. On current MacOS versions the file saved to your desktop is in png format (Mac OS X versions around 10.4 saved screenshots in .tiff format), but on OS 9 it’s in a less common .pict format.
By today’s standards the .pict file format is even more unusual as it uses Classic Mac OS file system features called a ‘resource fork‘ and a ‘data fork’. The issue with copying these .pict files from a Classic Mac OS filesystem to a modern file system is described here – when you copy the file you get the ‘resource fork’ but lose the ‘data fork’, in this case losing most of the image file data. When I tried this and viewed or converted the file on MacOS each of the files only has a section of the image, or none at all.
The trouble with this approach is if you’ve already copied the .pict files from OS 9 to a SMB network drive, you’ve already lost part of the file and it won’t convert as expected.
The best option as described in the first post is to convert to a jpeg or gif on OS 9 before moving elsewhere. I’ve seen posts suggesting to use Quicktime Viewer, but the version I have on mg G4 running OS 9.2.2 doesn’t have a Save As or Export feature, not that I could find anyway.
Instead what I found that worked for me was to download GraphicsConverter from Mac Garden here and use Save As changing the file extension to .jpg