Image has width and height properties, not "size"

Summary:
this makes it actually do what it was intended to,
set the source size to the actual thumbnail size to keep a scaled
down texture in menory

BUG:369670

Test Plan: normal usage, seems to work as expected, no warning

Reviewers: Plasma, sebas, davidedmundson

Reviewed By: Plasma, sebas, davidedmundson

Subscribers: davidedmundson, broulik, apol, plasma-devel

Tags: Plasma

Differential Revision: https://phabricator.kde.org/D2919