To reduce an image’s file size without changing its dimensions:

Choose Tools > Adjust Size.

Select “Resample image.”

Enter a smaller value in the Resolution field.
You can reduce an the file size of an image without reducing its dimensions, if you need to save disk space or download an image more quickly. The reduced image will contain less detail than the original.
Choose Tools > Adjust Size.
Select “Resample image.”
Enter a smaller value in the Resolution field.
The new size is shown at the bottom of the dialog.
To resize multiple images at the same time, display the images in the same window, select them in that window’s sidebar, and then choose Tools > Adjust Size.