So you've reached 300? Either way keep going! You will automatically get anything new released even if you passed that vote streak. I just hit 600 yesterday!
Not that hidden, well, the ore buster one was. The dragon one is right in front of you when you spawn in xD
You can always shrink it down using a tool like this one: http://www.shrinkpictures.com/ and make it a bit smaller. It should still be of good quality, as the highest accepted isn't actually that low. So I'd try some different sizes and see what is the best looking one that's still accepted. I did the same for the Lion King Piwi avatar :)