Want to make this?

Choose the image you want to put diagonal. Go to Layer >> New Layer (Shift+Ctrl+N)

Go to File >> New and make a new transparent image 4x4 pixels.

Choose a color you want for your diagonal lines. Click on the Pencil Tool (N). Draw a diagonal line left to right or right to left. Go to Edit >> Define Pattern and name your pattern.

Click on the Paint Bucket Tool (K). Go to the drop-down menu next to Fill, change Foreground to Pattern. Now fill the transparent layer with the pattern.

A good blending mode for the Diagonal Line layer is Soft Light, as what I did beloW. (reducing opacity is also a good idea)