You have to have saved up enough Gold to buy the items that enable images and links. Thumbnails are done by using both together.
Or you can make nice to me and I can give them to you. Or you can try to justify that you deserve them. I won't lie; I've given those items away before.
Revned (among others) thinks this system is rather controlling, but I like to deal with items and it makes more sense to me to give and take these items rather than enable it as a default and then have to deal with people who misuse them. I'm sorry, but "goatse" is not something I want to walk into every time I click into a topic. Sure, I exaggerate, but really, it'd be easy enough for someone to do. I worry enough about my site in other aspects it's one less potential headache. Or something.
Of course, now I'm going to get a bunch of PMs asking if they can have these items. >_> Either that or this topic will turn into a lobbying attempt to get me to change my stance...