Actually if your on your iPhone. Go to a thread and hit the top where the title is. It will take you to the website in say safari. Once there click the user pc
on the left hand side. Once in there you'll scroll down and on the left hand side you'll see under the misc section the word attachments.
Click attachments and select which pictures you wanna delete. You can start from oldest to newest.
I just did it. Go up to where the title is. It says App issues. Click on it. It will take you here
Top left hand side. You'll see user cp. click that and do as I stated above. Misc section down on left hand side. Then click attachments.
Lemme know if you need more help. But it can be done on your iPhone. I do it all the time and I'm never on the pc
This can all be done from your iPhone as I just did it.