In order to upgrade to Lion you need to get the Snow Leopard DVD from the Apple store since it is a prerequisite. You do not need Leopard.
Here are the system requirements for Lion and Mt Lion;
No. It is very unlikely that an iMac that came with Tiger can run Lion or Mountain Lion, and possibly not even Snow Leopard.
A PPC iMac can not upgrade beyond Leopard 10.5.8. As for an Intel iMac you can bypass Leopard and go straight to Snow Leopard, but:
Start by checking if you can run Snow Leopard:
Requirements for OS X 10.6 'Snow Leopard'
OS 10.6 Snow Leopard is once again available for $19.99 from the Apple Store:
and in the UK:
but nobody knows for how long it will be available.
To use iCloud you have to upgrade at least to Lion, but some functions are only available in Mountain Lion:
You can also purchase the code to use to download Lion (Lion requires an Intel-based Mac with a Core 2 Duo, i3, i5, i7 or Xeon processor and 2GB of RAM, running the latest version of Snow Leopard), or you can purchase Mountain Lion from the App Store - if you can run that:
When you have installed it, run Software Update to download and install the latest updates for Snow Leopard.