There must be some time delay set up between the time you post pictures as attachments and the time you can delete them. I'm not sure how long because I just figured this out today, but since I've only been a member for about 6 months and it seems that only my earliest picture posts allow me the option to delete them, we can estimate that it's 6 months at the most. Maybe it's something else, but this should be looked into so we know for sure.
Here's how:
1. Click Myrollitup. Located top right, next to Log Out.
2. Click Attachments under My Settings > Miscellaneous.
3. Click Show Thumbnails near the bottom of the page.
4. Click the check boxes that are available on the right side of the page.
5. Click Delete Selected.