ImageController: We should not add "directory separator" twice on "folder" var

On linux platform we obtain: myfolder//xxx.png
On windows platform we obtain: myfolder\/xxx.png, and it doesn't work!

We need only myfolder/xxx.png in any case
Merge pull request #255 from jpierront/master

 ImageController: We should not add "directory separator" twice on "folder" var