Ah. No, a screenshot that you make using the Command-Shift-3 or Command-Shift-4 key combinations is saved in PICT format — a file format that Simpletext can display, but not create. To convert it to JPEG format, open it in another application that can handle graphic files and do a Save As..., choosing JPEG as the file format.
Applications you might use for this purpose include Appleworks if you have it,
JPEGView (freeware), or any photo image editing program. To open a PICT file in an application other than SimpleText, drag and drop the file's icon onto the icon of the chosen application, or else open the application first, then use its Open... command and navigate to the file.
You'll need to include the .jpg extension in the new file's name so the PC will recognize its file type.