Okay, GIMP is a watered down photo editing program much like the popular Photoshop. If you don't already have Photoshop, GIMP should suffice for your needs. First open the TIFF image using Photoshop (file--> open--> select the tiff image) then go back to file and click "save as" raise the quality level to max (12) and on the bottom drop down menu of the window, select jpeg. Voila you got yourself a jpeg
Actually you can even do this using Paint ( a program supplied by microsoft with all versions of windows (Programs > Assessories > Paint). Paint can open (and save to) TIFF files and also convert and save to JPEG and various other formats. So you don't need to have a specialised photo editing software program to do this.