Add new image size for silver sponsors

[DirtyF]

In order to distinct sponsors type, we need to display the logo in different sizes.

Don' t know if it's the right way to do it ;)

[goulvench]

Your code only addresses half of the issue, don't merge right away because I'm about to commit a fix that does it all.

[DirtyF]

Thanks, didn't know how to update the loop. Good job !