CompuServe Messages

There's a shareware PC utility that will convert between many formats, including Targa and JPEG, called Graphics Workshop. They also have a Windows version. If you can't find it here, try some of your local PC BBSs. Also, have you tried hooking your PC and Amiga up via null-modem cable? You can transfer files at 19200 and it won't matter how big the file is – you won't have to downgrade to JPEG to transfer. I don't think it'll take much longer than converting to JPEG, writing to floppy, and reading on the PC.