YIS - There is an upload limit, but you are no where near it at the moment. You have used approximately 1/3rd of 1 percent of your allotment. That's why I was curious if you had seen an error message or some other reason you thought you had reached your limit.
Anyway, as you surmised, the reason you can't delete the attachments is that the thread containing them (the one you resurrected from the dead) had been closed. For several reasons, attachments can't be deleted from a closed thread. If you do need them to be deleted, reply here, and I'll delete them for you. Otherwise, let's leave them there since your posts mention the attachments.