So Email On Acid stole a theme from you guys (StampReady). Now, you want a quasi fool proof method to prove that you have been screwed over. You had a hunch that the images that they used are identical to yours. Only issue is, though, is that you only showed the meta data (the filename, size, etc.).
Here's one method that people on Hacker News was suggesting: checking the hash sum.
I'll be showing you guys how to check the MD5 sum to be exact.
Open up an instance of OS X's Terminal, and type out the following command:
if [[ `md5 -q yourfile.png` == `md5 -q theirfile.png` ]]; then echo "true"; else echo "false"; fi
In the above command,
yourfile.png represents the file that you created, while
theirfile.png represents the file that Email On Acid supposedly created.
So I suggest that you modify that above command according to how you saved those files.