Welcome to the guild ! Yes a lot of people post an image in their first message. And only those that are successful receive rep... I know the rules are more strict now than before so it might be a security issue.
Anyway a 4 mb PNG should not be too big from what I have seen so far. My PNG maps would be more around 70-100 mb. That's why I recommend not to use this type of image unless in some cases.
Also, 4mb image may take some times to upload.