System 7.x screenshots are created in PICT format (data file type) Mac files(computer system file format). Adding a suffix does nothing for file type conversion. It is still a PICT file in mac file format, and adding any non-pict suffix is useless. If you can get the file over to the PC universe properly a suffix added there may give a hint to a suitably capable graphics program to try reading it using PICT format. But the suffix does no such conversion by itself. MacLinkPlus I think will do the computer system file format bridging between Mac and PC world, PICT to PICT and JPG to JPG but not data file type conversion, and PC exchange should allow you to write PC format floppies if I recall correctly. Probably easier to do PICT to JPG conversion on the Mac side before transfer, programs like Photoshop and Image Alchemy come to mind, unless you know your Unix world graphics program accepts PICT. If PICT is one of the input options, then it is just a matter of getting the PICT into proper form for the PC/Unix world and onto compatible media. How are you transferring the files you are trying?

Edited post: I stand corrected by the manual, MacLinkPlus 7.5 will convert from Mac (PICT) to PC (TIFF, or Windows Bitmap.BMP, also PC Paintbrush.PCX, Windows Metafile.WMF, Wordperfect Graphic.WPG). I do not have data on other versions of MacLink.