Can't you just do this:
If you double click a png file "Eye of Gnome" - eog opens up. Do a save as - jpg and it becomes smaller in size. I think eog is installed by default, if not it's in Synaptic.
If your talking about actual size , as in x-y dimensions you could install nautilus-image-converter which adds "Resize Image" and "Rotate Image" to the context menu in nautilus.
Please add a [SOLVED] at the end of your original subject header if your question has been answered and solved.