How to select multiple pictures in Google Photos

There is no option to "select all". Ctrl+A will not select all.
You can give feedback to Google to request a way to select all photos in one go.

There are several ways to select multiple photos and videos though:

Select several consecutive images

The same actions can be used to select or deselect pictures.
The number of selected pictures does appear in the left upper corner.
Tap/click on the X to the left of the number to deselect all.

On a computer:

Photos taken on a specific day:

1. Hover the mouse over the date that appears above a set of photos.

2. Click on the check mark that appears to the left of the date.

Other range:

1. 1. Hover over the first picture

   2. Click the white circle with check mark at the top-left corner of the photo.
      The circle will turn blue with a white check mark.

   3. Scroll down not too fast, for example using the mouse wheel, arrows or Page Down keys

   4. Hold the Shift key and hover with the mouse over a thumbnail. When the thumbnails turn blue you can click.
      Now all pictures from the first to the last picture are selected.
      When you scroll too fast and the thumbs do not turn blue, you must scroll back, select one pic after the already selected ones, and continue. In order to go faster you can display more thumbs on a page by using Ctrl-. But note that some actions like downloading are limited to 500 selected photos.

   5. Displayed in the top left corner (in the white bar) is the number of photos that is selected. Click X to deselect all.

On a mobile device

Photos taken on a specific day or month:

1. Toggle between day or month view by spreading/pinching fingers on the screen

2. Tap on the check mark that is always present to the right of the date or month.

Other range:

1. Press the first picture till it is selected

2. Then keep the finger pressed and sweep down till the last one you want to select.

Select several pictures that are not consecutive

On a computer

1. 1. Hover over the first picture

   2. Click the white circle with check mark at the top-left corner of the photo.
      The circle will turn blue with a white check mark

   3. Click anywhere on another image to select it.
      Click again anywhere on a selected image to deselect

On a mobile device

1. Press the first picture till it is selected

2. Tap the other photos you want to select

Select pictures that were recently uploaded or added

On a computer

1. Click on "Explore" in the left menu

2. Scroll down till you see "Your activity" > "Recently added" and click on it.
   That brings you to https://photos.google.com/search/_tra_
   Photos will now be sorted by the date the photos were added to Google Photos

3. Use any of the above selection methods

On mobile devices

1. Tap on "Search" at the bottom of the screen

2. Scroll down till you see "Recently added", and tap on it.
   You can now toggle between day- and month view (spreading/pinching fingers)
   Photos are sorted by upload day or month.

3. Use any of the above selection methods