How can I add a gradient to images for clear text overlays?

[Jeffrey_Shelton]

We are in the process of giving the site a complete makeover. Our old theme had a slider than would easily allow me to brighten or darken a banner image so text could be easily read.

I have spent a few hours today reading CSS tutorials and trying to achieve this effect with multiple photo editors. Ive had no luck.

There has to be an easy way for me to add a gradient over the images to make text overlays easy to read. 

Does anyone have an easy answer or can explain what to do like I am 5?

[Jeffrey_Shelton]

Can anyone help?