Step 1: Duplicate your layer
Open your image in Photoshop
and duplicate it by going to Layer>Duplicate Layer or by dragging it onto the new layer icon in the layers palette. Then double-click the new layer's name to rename it; I've changed it to 'frame'.
Step 2: Draw your frame
Select the Rectangular Marquee Tool and use it to draw around the part of the image you want to be inside the frame. Next, go to Select>Inverse (Cntrl, Shift and I) to create your border.
Step 3: Blur your selection
Go to Filter>Blur>Gaussian Blur and adjust the slider to add enough blur so the shapes inside it are only slightly visible through it.
Step 4: Add a tint of colour
Frost/glass has a slight tint to it so while keeping your current selection active, create a new layer, rename it 'tint' and use the Paint Bucket Tool to fill the frame with your chosen colour. There's no specific colour you must use, however a blue/white tint works well if you're going for the frosted glass look. To bring the image back through change the opacity of the layer. You can also change the Layer Mode to Overlay but this isn't always needed.
Step 5: Finishing touches
Finally, you can add more blur to the colour you've just applied if you want to soften the image even more or try adding a drop shadow. Click here for information on how to do this. You could also add a line around the edge of the image by double clicking on the tint layer and selecting stroke.
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