What can I do to make my galvanized steel shed stronger?
In Florida, I have seen many sheds like the one in your picture further supported with cable or strap tie downs. Heavy galvanized eye bolts embedded in each side of the concrete slab. Then straps or cables attached to one eye bolt, up and over the roof and down to an eye bolt on the opposite side. Hope this helps, and hope you never need it.
You can get anchors similar to the ones pictured below at many home stores. They can be installed and then the slab poured over them for added strength.



Then steel cable or web tie downs can be fastened to the anchors. The web tie downs come in up to 50 foot lengths and steel cables in any length. They can go over the roof or attached to other structural points depending on the design of the shed.