Frequent error 403 when uploading photos


Since the migration to API 1.1, We encounter frequent troubles uploading the photos of our Twittes.
We search the last 5 twittes of an account then retrieve the twittes and the images linked.

The server do not send us any rate limit error but when trying to download the image of the twittes the server regularly returned an HTTP response code: 403 for URL:

Is there any rate limit when retrieving images ?
Some time, the problem is just sporadic and we have to wait a few minutes to download the image. But right now the upload has been blocked for 3 days.

Thank you for your help,