First you will need to read on how to make a template override.
After that, and after you have in your template the K2 files, you edit the category_item.php and the item.php. Then you will probably need to write some CSS.
For the images you will have to put the dimensions you would like in general parameters of K2 and on the category parameters to set which size (XS, S, M, L, XL) you would like to use for the listing and for the item page.