Ubuntu 16.04 & Firefox 46.0.1 just uploaded a jpg with no problems here.
The message you get after refresh is OK, it stops you accidentally resubmitting a completed form. ( Resending a form would be embarrassing if you happened to be paying a bill. )
The white screen bit is wrong though. As your screenshots uploaded I can only think there's something odd about your other jpegs. The website doesn't just copy pictures so it might be their fault – uploads get resized and the forum's software could be gagging on something.
You could try using 'convert' on the pictures to force regeneration of the file, for example from jpg to png and back again. From a terminal:
$ convert your.jpg new.png && convert new.png new.jpg
5 minutes later, bird successfully rescued from my daughter's vile cat.
PS : convert can resize photos as well. Minimising before sending has solved 'big picture' problems for me in the past.
Edited By SillyOldDuffer on 24/05/2016 17:31:17