So far this is the best short description I found on the net: (HERE)
"Flickr’s free account offers a 100MB monthly upload limit (10MB per photo) and only 3 sets (aka album) can be created with no monthly bandwidth limit. Flickr allows only the 200 most recent images to be viewed once you have reached more than 200 images uploaded. You can post any of your images in up to 10 images in a group. Original size images are saved for later upgrade and only smaller images will accessible after being resized.
As for Picasa Web Albums, a free photo sharing service offered by Google providing 1GB of free storage and the images uploaded to Picasa Web Albums can be no larger than 20MB and are restricted to 50 megapixels or less with no monthly bandwidth limit. The maximum number of web albums allow is 250 and the maximum number of photos per web album allow is 500.
Lastly, Photobucket’s free account offers 1GB of free storage with a 25GB of monthly bandwidth limit and custom URLS for up to 10 albums. The images uploaded to Photobucket can be 1MB each or by 1024×768 resolution. However, you can have up to 50 images per slideshow (some slideshows vary) and you are allow to upload a 5 minutes of video playing time (100MB in filesize)."
{Image via Weheartit}