Solved.
KB29850-Images are not displayed when browsing via BlackBerry Mobile Data System Connection Service routing through a HTTP proxy
RIM made a change to HTTP transcoding from MR6 onwards. Using workaround 2 in that Knowledgebase article resolved the problem immediately.
For this workaround you need to an older (pre-MR6) version of a file called transcoderHTML.jar.
To get this file:
- Go into the original install folder for BES 5.0.3 (C:\BlackBerry\BES 5.0.3 Full\besx_5.0.3\bundle0033 on our BES - I think this is the default path)
- Find the file called MDSCS.cab and open it
- Within the cab file find the file called File_22_40_transcoderHTML.jar.C60727D2_6BDE_4F06_B 932_CA2A75909E6B and extract it
- Rename the extracted file to transcoderHTML.jar
You can then follow the instructions in the KB article to replace the newer file.