you have a few option:
if background is white :
rgba(255,255,255,0.3)
color used in a gradient on top of image
#alpha_wrapper {
width: 540px;
height: 360px;
float: left;
position: relative;
color: #fff;
}
#alpha_2 {
background: linear-gradient(rgba(255, 255, 255, 0.3), rgba(255, 255, 255, 0.3)), url(https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png) no-repeat 0 0;
width: 540px;
height: 440px;
}
#text_holder_2 {
color: red;
font-weight: bold;
font-size: 28pt;
position: absolute;
top: 20px;
left: 20px;
width: 500px;
}
<div id="alpha_wrapper">
<div id="alpha_2">
<div id="text_holder_2">
THIS IS MY TEXT
</div>
</div>
</div>
rgba(255,255,255,0)
color used as an inset shadow:
#alpha_wrapper {
width: 540px;
height: 360px;
float: left;
position: relative;
color: #fff;
}
#alpha_2 {
background: url(https://www.google.com/images/branding/googlelogo/2x/googlelogo_color_272x92dp.png) no-repeat 0 0;
width: 540px;
height: 440px;
box-shadow:inset 0 0 0 270px rgba(255, 255, 255, 0.3);
}
#text_holder_2 {
color: red;
font-weight: bold;
font-size: 28pt;
position: absolute;
top: 20px;
left: 20px;
width: 500px;
}
<div id="alpha_wrapper">
<div id="alpha_2">
<div id="text_holder_2">
THIS IS MY TEXT
</div>
</div>
</div>
If the background is not white, but has as a main color or plain color , you can take a look at : http://codepen.io/gc-nomade/pen/wouBe for more examples.
Else you can turn your image into a png and add the transparency to the image itself and not relay on CSS