For reference, in your usercp page, the last option in the left-side menu is attachments -- from there, you should be able to see and delete all attachments you've ever made.
There's no pressing need to delete attachments, but keep in mind that attachments currently represent about 6 GB, and the vast majority of those nobody will ever look at again.
Personally, I find it easier to upload images to imageshack or photobucket or or imgur or some other web photo service. That allows management of all online images in one place, rather than having to manage images on each site separately.
Those megasites also allow more efficient use of resources, since they can hash each image and deduplicate it across their entire image database (if you and I were to upload the same image, they would only need to store it once). It truly is an economy of scale.
vBulletin cannot do that. It will not let you upload an image identical to one that is already exists, and if you link to an image that already exists in the vbulletin database, you are at the mercy of the image's owner not to delete it.
Imagine a pathologically idealistic, anti-religious, culturally-relativistic, iconoclastic, socially-irreverent, sociologically-progressive signature line here. Or don't.