Image widget removing transparent background

Hi, Since updating to WordPress 3.5 Im having an Image Widget issue which was not there before.
When inserting an image into image widget which is a png with a transparent background. All is fine at first sight but upon clicking ‘Save’ in image widget instead of the transparent background image merging into the background color, a white box appears see https://learnfastuae.com/learning-ability-coaching/end-school-struggles/ Notice the ‘Free Seminar’ image with the white box.

Whereas here
https://learnfastuae.com/ on the homepage the same ‘Free Seminar’ image is fine.
Why is wordpress image widget taking out the transparency ???
Please help,
Thanks