To demonstrate this effect, I am going to use this image:

Open up the image and copy and paste it onto a new layer by pressing the copy button and then going to Edit >> Paste >> As New Layer

Using the menu along the top, go to, Effects >> Blur >> Gaussian Blur

A box like the above should open. Set everything like I have. (decrease the numbers to alter the strength of the blur)

Your image should now have an effect like this:

Now Got to Layers >> Properties and set the opacity to 30

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