Like you, mindimer, I got a similar or maybe the same url error. Part of the trick appears to be that the plugin unpacks in a directory called 'ecards'. Nice and tidy. Unfortunately, the plugin hasn't got the directory part sorted out. I can get past the upload stage by editing some of the lines with the full path.
This'll be slamming it with a sledgehammer, there'll be some elegant php way of doing this, meanwhile, I edited lines 146, 150, 163, and 174 i.e. where ever there's path reference, and added the directory (ecards) and told the plugin where stuff is to be found.
E.g. wp-admin/edit.php?page=ecards.php became
http:/ /myblog/wp-admin/edit.php?page=http:/ / myblog/wp-content/plugins/ecards/ecards.php
Revolting, but suddenly I could upload images, plonk the [Ecard=17] (or whatever number it was), send the email, receive the email, and delete the image file.
Note, I haven't mentioned I could pick up the card yet, or that the image file was attached, or quite any part of that. But at least lumps of the plugin are starting to work. There will be a vastly more elegant method of getting the path stuff sorted - I'm a php-newbie and haven't got the basics sorted yet.
I'll continue to hammer away at it, it can't be that complicated. Please give it a shot and report back how it works. Once we can get it going, we can turn our attention to the more important task of making it look good.