That is a problem...you should have saved or copied photos to phone storage,sd card, flash drive or PC before deleting them from picassa. Picassa is a cloud server for photos and we use it for back up of photos so they don't get lost. You can disable syncing to picassa or auto upload to picassa and save new photos to phone or sd card only if you choose but, if you delete the last available of the copy from the cloud it may be gone forever...if you have no other copies you may be able to contact google/picassa to see if an undo of the delete is possible. Then copy images where you want them to be before deleting them again.
If any other cloud backup for your device was enabled you may have a chance of finding the pics there but I am unsure.