This FAQ discusses a method of deleting multiple photos in multiple folders that provides better control than the Photo Tray over selection of photos.
WARNING: This process will delete photos from your computer!
This method uses the Search features of Picasa and the ability to tag photos with a "Star" so you can find them later. The star button is located on the bottom middle area of Picasa. If you use the Star to mark your best photos, you need to be extra careful that those starred photos won't be deleted in the process described below. Instead of starring your best photos, you can use a text tag - see Using Text Tags for Selecting Photos below
In this method you assign a Star tag to each photo you want to work on, then you filter so you are only working with photos that have been starred, then perform the operation on all the photos in all the folders at once.
Before starting, go to the top of Albums collection in the left column and make sure the green Starred Photos album is not there (near the top), because any photos already starred will be included in the photos you are selecting.
Go through the folders you want to select photos from and "Star" any photos you want to work on. There are several ways:
You can just select photos in the library and click the white star in the bottom center above the green "Upload to Google Photos" button, or
Double-click the first photo in the library to go into edit mode, then step through the photos in the edit screen using the left and right arrow keys and click the star button above the green share button for each photo you want to select, or
You can also start a full screen slideshow then click the star on the photos you want to select.
When you have finished adding a star to photos, click the green star filter button at the top center of Picasa. This will filter will show only starred photos.
At the top of the Albums collection in the left column, find and click on the green Search Results album.
The search results album now has all the starred photos and the album is selected.
Select the photos you want to delete as follows:
To select all of them, go to the Edit menu and click "Select All" or Ctrl+A
Select the photos you want to work on in the next step using the normal Picasa methods such as Ctrl+click
After the photos from multiple folders are selected as described above they can be deleted by any of the following actions:
Press the delete key on your keyboard.
Go to the File menu and select "Delete From Disk"
Right-click on one of the selected photos and select "Delete From Disk"
In this method you assign a Text tag to each photo you want to work on, then you filter so you are only working with photos that have that tag, then perform the operation on all the photos in all the folders at once.
Before starting, note that any photos already tagged with that tag will be included in the photos you are selecting.
Go to the right column in Picasa and click on the 3rd button below the column to get to the tag function. (it has a yellow tag icon)
Select a Photo you want to tag for deletion, then type the tag you want to use to flag the photos into the new tag box at the top of the Tags column.
The tag must be unique so it won't mistakenly appear in other photos. (I use something like XXbadXX)
Click the + button to the right of the tag entry box and the tag will be applied to that photo and will also be assigned to one of the "Quick Tags" buttons at the bottom of the right column.
Go through the folders you want to select photos from and click the appropriate quick tag button for any photos you want to work on.
When you have Tagged all the photos in all folders that you want to select, then go to the search box at the top center of the Picasa screen.
In the search box type the following: tag:YourTag without any spaces. (For my tag above this would be: tag:XXbadXX ). This will filter so you only see the tagged photos.
At the top of the Albums collection in the left column, find and click on the green Search Results album.
The search results album now has all the tagged photos and the album is selected.
Select The photos you want to delete as follows:
To Select all of them, go to the Edit menu and click "Select All" or Ctrl+A, etc.
Select the ones you want to work on in the next step using the normal Picasa methods like Click, Ctrl+Click, etc.
After the Photos from multiple folders are selected as described above they can be Deleted by any of the following actions:
Push the Delete Key on your keyboard.
Go to the File menu and select "Delete From Disk".
Right-Click on one of the selected photos and select "Delete From Disk".
Notes:
The Search Results green album is a special album where you can delete photos from multiple folders.
You can't use the green Starred Photos album to delete photos because it is a normal album and doesn't allow delete from disk (only remove from album).
You can't use the regular blue albums to delete photos because they are normal albums and don't allow delete from disk (only remove from album).