Read how to resize images in DJ-Catalog2:
Open DJ-Catalog2 component.
Open component's parameters/options
Go to "Image Processing" tab
Change the values (you can change the values globally and also apply different for Category and Producer settings) and save your settings.
Go to "Images Manager" tab and use one of "Recreate thumbnails" buttons, depending which image sizes you want to change
(since ver. 3.7)
In order to compress images generated by DJ-Catalog2 - that is thumbnails created right after adding new images as well as thumbnails dynamically created by modules - you simply need to click on "Optimize images" button. The console will show you which images has already been processed and the reduction rate next to each.