Hi,
You can customize the size of the pictures in the config under the tab files. Please note however that the images are redimensioned only at upload and that thumbnails will be automatically generated if they don't exist (so, if you clean the thumbnails folder inside the file upload folder, they will be recreated with the good size).
You can add several images no problem for that. on listings of products, only the first picture will be displayed, but on the product page, you will be able to see all of them.