Photos that you have receive via Gmail are physically stored at the Google Gmail servers. Picasa is a program that displays photos that are on your computer's local disks. If you want Picasa to display the photos you receive in Gmail, you have to download the photos from the Gmail servers to your computer's hard disk. Once the photos are on your computer's hard disk, Picasa can see them and let you work with them.
In Gmail, you have two options for downloading pictures:
Click the "Download" icon
Click the "Download all attachments" icon
If you "Download all attachments," the downloaded file on your computer will be a "zip" file. You'll have to unzip it in Windows File Explorer. Right-click the zip file and choose "Extract" from the pop-up window. After the files are unzipped, you may delete the zip file. Your pictures are now in a folder in Windows File Explorer.
To make the pictures display in Picasa, the pictures need to be in a folder that is being scanned by Picasa. Under Tools > Folder Manager is where you will select which folders you want Picasa to scan. Choose "Scan Always"