Tiled Galleries: use alt value instead of title for alt attribute

I added alt attributes to Tiled Galleries in e75fecaa818a3c84fcc9b64e729fe300bad869c0
but should have use the actualy image alt instead of the image title
Merge pull request #193 from jeherve/tiled-alt-images

Tiled Galleries: use alt value instead of title for alt attribute