ReSize JPGs

[George Gambill]

I Have two pictures (.jpg) which were taken at 1600 x 1200.  They want to
print (by default) huge (as in greater than 8.5 x 11).  I want to print them
smaller.  I would like to resize them to 800 x 600 or 640 x 768.  

I can print them on 8.5 x 11 (or smaller) via software that came with my
Cannon S750, running under control of Redmond and Co.  I want to use Linux.
I want to get away from the regular feedings required by those Redmond
folks.

First and for most, what Linux programs are available to resize the jpg
and/or convert to another format?  Resizing will allow me to email them back
to the grand daughter so she can print them also.

Second, is there a  better format than jpg?

Third, is there a better way to print smaller with Linux.

Thnks, George