You can use this link to fetch all of them:
However, it still won't be too reliable as it contains also extensions which are no longer available on Magento Connect site.
You can also fetch list of releases for single extension eg:
And then extension release details:
There are some descriptions and other data there, however it doesn't include url or image.
FYI, all this details come from
Mage_Connect_Rest class which exist in each Magento release.