When you upload files in the File manager, you can press the CTRL key to select several files or images at once.
Select the first image with a left mouse click.
Then press the Ctrlkey and hold it down.
Click next on the other images you want to select (while still holding the Ctrl key down).
Once all the images are selected, release the Ctrl key.
An alternative solution consists in holding the mouse left click down and to drag the cursor to draw a selection "square". All the images within the "square" will be selected when you release the left click.