How to add a transparent background to an image in gimp

I would need to make it a little more watermark like before I could use it as a background image otherwise it could make it hard to discern any text on the page. To turn the image into a “watermark” I’d need to make it partially transparent by altering the image’s opacitysetting in GIMP. Here’s how to do that:

• open the image with GIMP,
• use the top menu bar to browse the Windows menu and move your cursor downwards to Dockable Dialogues then select Layers. Alternatively, press Ctrl+L,
• the Layers dialogue box has an Opacity slider control. Ensure your image is highlighted then move the slider to your desired opacity level (50% is generally good enough),
• next we flatten the image by right-clicking over the image, moving the cursor to Image then selecting Flatten Image. We do this to ensure that the opacity is preserved when we save the image as a jpeg, gif or png etc.
• Voila! Save the image.

Taken from http://journalxtra.com/easyguides/how-to-make-a-semi-transparent-image-using-gimp/#Make-a-Complete-Image-Semi-Transparent