If you mean photoshop (from: http://www.alexjudd.com/?p=5):
But another easier solution may be possible. Edit the contentslider.css file here in two places (additions/changes highlighted):
Easy – go into Photoshop and adjust the levels to remove the true black in the image – Image -> Adjustments -> Levels and slide the true black slider 1 or 2 pixels to the right. Save out your image again and the jpg artifacts are gone.
Give that a shot first. If it works it will save you a lot of editing of images.
position: relative; /*leave as is*/
overflow: hidden; /*leave as is*/
border: 10px solid navy;
width: 400px; /*width of featured content slider*/
visibility: hidden; /*leave as is*/
position: absolute; /*leave as is*/
left: 0; /*leave as is*/
top: 0; /*leave as is*/
width: 390px; /*width of content DIVs within slider. Total width should equal slider's inner width (390+5+5=400) */