the problem is that I don't understand the SVN properly.
What about it don't you understand? We can try to help.
I can't help you with your Mac issues, though, except to suggest that perhaps the command line svn might be simpler? Generally speaking, with SVN, you have a local copy on your system and you change it as you see fit, then you commit all the changes at once. No deleting of anything online, you're not editing things online. You edit them offline and then synchronize with the SVN repository.
PNGs are not smaller, JPG compresses better, at least the filesize is a lot bigger when I save-as PNG where JPG would suffice. I use PNG only when I need 24bit transparency.
PNGs should be much, much smaller for screenshots, because PNG is good at compressing large blocks of single colors. But for photographs, you are correct, JPG is better.