If you have Windows 98 first edition, you have all you really need.
Start
Run
mspaint <enter>
Open your picture, then menu item image, flip/rotate... select rotate by angle, select 90 degrees and press OK
That should do it for you. Then File, save.
Or with keyboard shortcuts:
Windows key + R
mspaint <enter>
Ctrl + O
Select your image
Ctrl + R
Down arrow twice
Enter
Ctrl +S
If you have Windows 98 SE, I don't think mspaint will open jpegs. I don't know why though.
Hope that helps