I seem to be having the same problem and looking at the F12, I see no errors.
My install is a WP 3.4.2 running multisite
Response
{“ok”:true,”data”:{“file”:”\/xxxx\/xxxx\/public_html\/wp-content\/blogs.dir\/1\/files\/2012\/11\/Coffee.gif”,”url”:”http:\/\/xxxx.com.au\/charlottes\/files\/2012\/11\/Coffee.gif”,”attach_id”:2842},”more”:null}
Headers
Request URL:https://xxx.com.au/charlottes/wp-content/plugins/image-pro-wordpress-image-media-management-and-resizing-done-right/src/request/upload.php?name=Coffee.gif
Request Method:POST
Status Code:200 OK
Request Headersview source
Accept:*/*
Accept-Charset:ISO-8859-1,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:en-GB,en-US;q=0.8,en;q=0.6
Connection:keep-alive
Content-Length:61455
Cookie:wordpress_4fb2d9f9500f00d498e28a5eaea8a96b=xxxx%7C1353410994%7C8424f7529f68ae3478bf4e843c04d214; wordpress_4fb2d9f9500f00d498e28a5eaea8a96b=xxxx%7C1353410994%7C8424f7529f68ae3478bf4e843c04d214; wordpress_logged_in_4fb2d9f9500f00d498e28a5eaea8a96b=xxxx%7C1353410994%7C61296f16e8fcdfdf4fd7a226ddd80957; __utma=40694206.72867421.1352027432.1352530169.1352722525.11; __utmz=40694206.1352027432.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none); PHPSESSID=ffc7dbebddf0f139ddef42036f5d5ec5; wp-settings-1=editor%3Dtinymce%26align%3Dright%26imgsize%3Dmedium%26hidetb%3D1%26mfold%3Do%26wplink%3D1%26galcols%3D2%26cats%3Dpop%26woosidebarsshowadvanced%3D1; wp-settings-time-1=1352806210
Host:xxxx.com.au
Origin:https://xxxx.com.au
Referer:https://xxxx.com.au/charlottes/wp-admin/post.php?post=300&action=edit
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.11 (KHTML, like Gecko) Chrome/23.0.1271.64 Safari/537.11
Query String Parametersview URL encoded
name:Coffee.gif
Response Headersview source
Cache-Control:no-store, no-cache, must-revalidate, post-check=0, pre-check=0
Connection:Keep-Alive
Content-Encoding:gzip
Content-Type:text/html
Date:Tue, 13 Nov 2012 11:32:47 GMT
Expires:Thu, 19 Nov 1981 08:52:00 GMT
Keep-Alive:timeout=2, max=95
Pragma:no-cache
Server:Apache
Set-Cookie:woocommerce_items_in_cart=0; expires=Tue, 13-Nov-2012 10:32:49 GMT; path=/
Transfer-Encoding:chunked
Vary:Accept-Encoding
X-Powered-By:PHP/5.3.13