Hi,
It is simple. You can combine a background-image and CSS3 gradient on the same element. Please try the below code.
.class
{
background-repeat: no-repeat;
background-size: 35px 35px;
vertical-align:middle;
margin:20px 0 20px 0;
padding-left: 45px;
display: block;
height:35px;
padding-top:5px;
color: white;
font-weight: bold;
font-size:18px;
z-index: 100;
background: url('https://s32.postimg.org/3ks1zaggl/lightbulb.png'), -moz-linear-gradient(45deg, rgba(0,0,0,1) 0%, rgba(0,128,128,0) 100%);
background: url('https://s32.postimg.org/3ks1zaggl/lightbulb.png'), -webkit-gradient(linear, left bottom, right top, color-stop(0%, rgba(0,0,0,1)), color-stop(100%, rgba(0,128,128,0)));
background: url('https://s32.postimg.org/3ks1zaggl/lightbulb.png'), -webkit-linear-gradient(45deg, rgba(0,0,0,1) 0%, rgba(0,128,128,0) 100%);
background: url('https://s32.postimg.org/3ks1zaggl/lightbulb.png'), -o-linear-gradient(45deg, rgba(0,0,0,1) 0%, rgba(0,128,128,0) 100%);
background: url('https://s32.postimg.org/3ks1zaggl/lightbulb.png'), -ms-linear-gradient(45deg, rgba(0,0,0,1) 0%, rgba(0,128,128,0) 100%);
background: url('https://s32.postimg.org/3ks1zaggl/lightbulb.png') no-repeat, linear-gradient(45deg, rgba(0,0,0,1) 0%, rgba(0,128,128,0) 100%);
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#008080', endColorstr='#000000',GradientType=1 );
}
Hope this will help you :).