Definitely not SmugMug on the cheapest, as they charge you for the service. If you use Picasa for your image management on your computer then the web upload bit is quite convenient. I'd guess that there are plug-ins for Picasa for other services too (I know there is for SmugMug), but I'm not sure about which ones there are. Picasa does limit you to 1GB on their server.

Also consider if you are looking to use it as a permanent repository or just a display source. There are some considerations in terms of total storage space, etc.

Ultimately the answer is that "it depends." There are a few other threads here about best service for hosting your images. A quick search should turn up some of them.

S.