We load all extensions from the official TYPO3 TER. Hava a look at https://typo3.org/extensions/repository
If you are an Extensiondeveloper, you should use a modern svg icon. On typo3.org only gif icons are working. So the best way at the moment (2016) is to upload a gif and a svg. Then typo3.org shows your gif and we (and also the TYPO3 Extensionmanager) use the svg icon.
We are happy about, if you help us to improve our data. You can do this by registering and login and use the suggest form for extension comparisons and ratings. If you want to do more (categorization, add Screenshots, write descriptions), get in contact with us: email@example.com - please write in german or english.