This is the best option IMO. But limit it to Full Members and above (in line with the avatars on profiles requirement), and the maximum image size has to be like 500KB or something.
The forum has so much money to pay for the storage costs of all those images, but there has to be some sort of daily limit for the number of images each user can upload, regardless of rank.
You have a nice point and of course a considerable aim too however there are a couple of things you need to look into first because an action that would limit any rank less than full members from posting images wouldn't be nice on newbies that have the true intent of giving quality to the forum as well as also benefit from the forum. In my opinion I suggest that the image limitation should be left as it is altering it will have a negative effect on the forum as well.
I don't really have much technical idea on how hosted websites are managed, however I think to prevent DDos attacks like this, both TalkImg and BPIP should implement a captcha that must be solved on the landing page before you can proceed to uploading an image. The rate at which DDos attacks are becoming frequent on the forum is quite annoying if they all implement captchas the only disadvantages may be from those using secure VPNs like Tor.