After additional investigation with Roan.

Revised steps to reproduce:
 
1. In test2 in FF - make sure that you just switched fro Read mode to Edit, or just refresh the browser window.

2. Insert - Media, type 'artist'(without quotes) to display search results.

3. In Console panel you'll see: 

Image corrupt or truncated: https://upload.wikimedia.org/wikipedia/commons/d/db/Artist_drive.JPG


In Network panel you'll see that instead of a thumbnail img - 112x149 and https://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Artist_drive.JPG/112px-Artist_drive.JPG, 

the request was made for for 1536x2048 img and
GET  https://upload.wikimedia.org/wikipedia/commons/d/db/Artist_drive.JPG

The Staus code: 206(not always).

Request/Response headers:

Request headers:
Host: upload.wikimedia.org
User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.9; rv:33.0) Gecko/20100101 Firefox/33.0
Accept: image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate
Referer: https://test2.wikipedia.org/w/index.php?title=ET_page&redirect=no&veaction=edit
Cookie: GeoIP=US:San_Francisco:37.7898:-122.3942:v4
Connection: keep-alive

Response headers: 
Accept-Ranges:"bytes"
Access-Control-Allow-Origin:"*"
Access-Control-Expose-Headers:"Age, Date, Content-Length, X-Content-Duration, X-Cache, X-Varnish"
Age:"1652"
Connection:"keep-alive"
Content-Length:"2115604"
Content-Range:"bytes 32419-2148022/2148023"
Content-Type:"image/jpeg"
Date:"Thu, 23 Oct 2014 23:43:46 GMT"
Etag:"6eaae0b7e042739bd4a04e686eab8dea"
Last-Modified:"Mon, 07 Oct 2013 01:54:56 GMT"
Server:"nginx/1.1.19"
Via:"1.1 varnish, 1.1 varnish"
X-Cache:"cp1048 hit (2), cp1048 frontend miss (0)"
X-Object-Meta-Sha1base36:"8x65i07x23pfu6j7h8yl33asimaupwx"
X-Object-Meta-Sha1base36:"8x65i07x23pfu6j7h8yl33asimaupwx"
X-Timestamp:"1381110895.66655"
X-Trans-Id:"txc9084137ff28405983d9c-0054498c3f"
X-Varnish:"431922781 428031845, 3387585776"


If you attempt to insert again the same image -  Request URL will be correct:
https://upload.wikimedia.org/wikipedia/commons/thumb/d/db/Artist_drive.JPG/112px-Artist_drive.JPG