When we say "K2 images", we mean the image that your upload via the image tab in the item edit form.
There are various reasons why we convert "K2 images" to JPG:
From speed (low file size), to performance (no disk IO checks, no database records for filenames, less memory for GD to resize the pic), to native restrictions in the PHP conversion library (GD) which really is still crap at resizing PNG-24 images...
If you want to use "transparent" PNG images (PNG-24), don't use the "image" feature in K2, but instead upload your image via the WYSIWYG editor (TinyMCE, JCE, JCK Editor etc.).