The $75 is an arbitrary number and isn't intended to ever be bought for that... There just is no mechanism currently in our digital goods marketplace to disallow selling to non-premium members. It's intended just as a benefit for premium members.
Premium membership is $25 for a single month, can be canceled right away and also gives you access to the other addons I made that are premium member benefits (for example the master/slave database setup). There is no end date on anything, and what you get during your premium membership month can be used indefinitely.
If a year from now (or whenever) there's a new version that you want because it has some awesome feature, you *would* need to be a premium member to get the update. But either way, the $25 cost for a single month is an indefinite license for as many sites as you own.
See this post for info on enabling error logging to your Server Error Log...I enabled this add-on and the externally-hosted images wouldn't display. When I disabled the addon they displayed again. I checked the image/proxy URL and it appeared to be as intended.
What do I need to enable in php to allow the images to download?
See this post for info on enabling error logging to your Server Error Log...
It's going to proxy anything that uses the [IMG] BBCode... although I suppose it would make sense to not proxy stuff on the same scheme/hostname.