I agree with Jonathan that if they are direct camera transfers, the the camera was not set to save them as jpeg. The same would hole true of they are scanned images as many scanners save at 'tiff' as their default.
The software that came with your camera should have a way to convert them from tiff to jpeg. Or, Adboe Photoshop or Ellements (cheaper for home users) should work.
If you have many, make sure that you try to find the 'batch' conversion as manually converting can take some time if there are many.