Online storage for the volumes that most photographers generate isn't there yet in most places on the planet. If you want to backup meaningful amounts of data (RAW or TIFF in particular) then issues like your upload speed, cost charged by your isp per Mb of upload need to be calculated. Then there is the annual cost of storage. Dropbox charges $99 per year for just 100Gb. You might need that much every month. If you don't want to trust having your valuable data committed to just one cloud destination, then at least duplicating/syncing to two destinations will be a must for many photographers.
Places worth checking out would be Adobe Creative Cloud which offers free storage (not sure of the max amount before you need to pull out your Visa card). There are plenty of other paid & free cloud destinations...a Google search will reveal them quickly for you. Google Drive and Microsoft SkyDrive are the first to come to mind. There are plenty of others.
If the need to access your files from anywhere is your prime motivator, what I have done may be relevant. We have a 15Tb NAS which sits on the network. It's in a very secure room with power backup UPS and two very strong levels of surge protection. It's a Synology system. Look 'em up. This is top-shelf stuff, very configurable.
If you have a static IP you can set up your own local cloud. Not only can you sync your current projects in real time to the NAS, you can set it up to be able to login to it from anywhere. Less and less now do I deliver clients their jobs on DVD/CD. We have set up a password protected share folder on the NAS and just email clients the URL to their project. Once they have downloaded their stuff and confirmed it's OK, you can take it down or leave it up for an agreed period. For a lot of smaller projects, we deliver now using
https://www.wetransfer.com/
Uploading to the NAS happens at very fast LAN speeds, orders of magnitude faster than any ADSL upload, and faster than cable. If you're lucky enough to live somewhere with latest tech fiber-optic then lucky you. ADSL download speeds are always much much faster than upload speeds, so your client can pull down a 2Gb job pretty quickly.
HTH...
-PW