It's because you're still a newbie. On bitcointalk, there are multiple ranks. Once you've gained enough activity you level up to the next rank. You have to be a jr.Member (30 activity) to be able to post images

You can also choose to use a faster method: Paying a small fee to get the  copper membership .

Details benefits of copper membership :https://bitcointalk.org/index.php?topic=2385104.msg24371150#msg24371150
(Copper - 0.00208333 BTC: Allows you to post images even if you are a newbie, reduces the time-between-actions limit, and provides some of the same benefits as being of natural Member rank)


From here you can buy the membership:

https://bitcointalk.org/index.php?action=credit;promote