Shotwell (mentioned by @Goddard) is a great replacement alternative for Picasa. Anyway you may miss some useful tools from Picasa.
This question has been asked before and I am not sure if you are referring to a new version of Picasa but as far as we can say, Picasa 3.9 can be installed by using this workaround.
I provide you with some screenshots of Picasa running in my Desktop via Wine (remember, Picasa won't run native in Ubuntu but you can run it via Wine which needs to be previously installed).
May I also suggest you to take a look in Software Center and run a search for "Picasa" (without quotation marks) and you will be provided with some alternatives from the official repositories as shown in the next screenshot:
Additionally, please consider to tell us which version of Ubuntu you are running in order to provide you with further details.
They have already started closing your question. so...