Virtually Unlimited? Yes, that means still Google offers users to buy extra space on Picasa but if you know the trick, you can upload unlimited photos and videos on this platform. According to the new ‘Free Storage Limits’ provided by Google, if you have signed up to Google+ then you can upload photos up to 2048 x 2048 pixels and videos up to 15 minutes without worrying about the storage limit as those photos and videos will not count towards the free storage of 1GB. Yes, that means even if you uploaded hundreds or thousands of photos and videos in that limit, your 1 GB free storage will remain available to you. It means you can upload unlimited photos and videos on Picasa without buying extra storage.
Then you must be thinking why Google is still having the sales page of extra space on Picasa. The answer is there in the question itself as if you (Google+ user) upload photos of size more than 2048 x 2048 pixels or videos of length more than 15 minutes, those photos will count towards the 1 GB free storage and your free storage will get consumed.
For non Google+ users, the options and procedure is the same but the limit for photo size is 800 x 800 pixels only. That means you can upload unlimited photos of size up to 800 x 800 pixels. Photos of size more than that limit will count towards the free storage limit. Videos length limit is same for Google+ and non-Google+ users.