A little background info…
Picasa (wiki) was and still is a great and simple desktop photo organizer and editor (as opposed to cloud solutions like Google Photos). Until recently you could log in with your Google (Gmail) account and send your photos as email attachment right from the desktop app (with Picasa automatically resizing large photo files).
Why can't I login or send photos anymore?
The Google login part of the app stopped functioning in May 2018. You may experience blank login screen or HTTP error 0 -  when you try to send photos.
So how do I fix this?
We'll set up an external email client (eM Client) with a "send only" Gmail account and set Picasa to use it for sending photos. It's a bit more involved process but the end result pays off.
Part 1/3 Install eM Client
- Download and run eM Client installation (BTW a great desktop e-mail client).
- On the last installation screen uncheck Run at Windows startup (you probably don't need it)
- In Windows 10 the setup should open the Default apps window. Select eM Client for E-mail there.
- When the eM Client is started the first time, cancel the Set up an account step. Cancel the Import data from other apps step if you get it first.
Part 2/3 Set up eM Client
- At the right of the window header click Menu → File, check Work offline (so it won't start downloading your messages while you're setting it up). Check there's a grey Offline box at the lower left before you proceed.
- Now click Menu → Tools → Accounts
- Enter your Gmail address.
- Fill in your name, click Next and keep only Mail and Contacts checked.
- Click Finish, a browser page opens, confirm Google account access there.
- Click Menu → Tools → Accounts → IMAP tab, delete Host name.
- Click Menu → Tools → Settings
- In General section uncheck Show window when an error occurs.
- In Mail → Send section uncheck Save copy of sent messages to Sent folder
- Finally, click Menu → File, uncheck Work Offline (i.e. disable offline mode so the app can send your messages but won't download you Gmail inbox because of the disabled IMAP host).
Part 3/3 Set up Picasa
- Click Tools → Options → E-mail tab and select Use eM Client in Email programme section. Option not available? ▼
- While you're there, bump up the Size for multiple pictures to 1600 pixels (for a better quality while still being small size).
Now you can e-mail photos from Picasa as usual. You don't have to start eM Client, it will open automatically. And you've added text formatting and spelling correction to sending photos from Picasa.