I have done an audit of my own posts & deleted some pics that are now a bit old hat.
They ranged from a few Kb to around 1.5Mb. It's the bigger ones I got rid of - saving 40 Mb on my own !!.
Perhaps you could do the same with yours
It's within your user control panel tools & you can sort by size if you click on the size column title.
The board has around 500 Mb of photo space which was full - hence the problem.
To control this I've reintroduced the sizing controls that Werner removed a while back.
Photo limits are now - 512Kb (0.5 Mb), 1024 x 768 pixels, limit of 6 per post.
Less is more... At 512K, the 40 Mb space I've created is about 80 photos, rather than the 20 it hosted previously.
This will likely mean that photos taken on recent cameras will need resizing down a bit.
You PC / Mac / or phone can do this quite easily.
Rgds
Matt