CSS2

If you need to make the image bigger, you must edit the image itself in an image editor.

If you use the img tag, you can change the size, but that would not give you the desired result if you need the image to be background for some other content (and it will not repeat itself like you seems to want)...

CSS3 _{^{unleash the powers}}

This is possible to do in CSS3 with background-size.

All modern browsers support this, so unless you need to support old browsers, this is the way to do it.
_{Supported browsers:}
^{Mozilla Firefox 4.0+ (Gecko 2.0+), Microsoft Internet Explorer 9.0+, Opera 10.0+, Safari 4.1+ (webkit 532) and Chrome 3.0+.}

.stretch{
/* Will stretch to specified width/height */
  background-size: 200px 150px;
}
.stretch-content{
/* Will stretch to width/height of element */
  background-size: 100% 100%;
}

.resize-width{
/* width: 150px, height: auto to retain aspect ratio */
  background-size: 150px Auto;
}
.resize-height{
/* height: 150px, width: auto to retain aspect ratio */
  background-size: Auto 150px;
}
.resize-fill-and-clip{ 
  /* Resize to fill and retain aspect ratio.
     Will cause clipping if aspect ratio of box is different from image. */ 
  background-size: cover;
}
.resize-best-fit{
/* Resize to best fit and retain aspect ratio.
   Will cause gap if aspect ratio of box is different from image. */ 
  background-size: contain;
}

In particular, I like the cover and contain values that gives us new power of control that we didn't have before.

Round

You can also use background-size: round that have a meaning in combination with repeat:

.resize-best-fit-in-repeat{
/* Resize to best fit in a whole number of times in x-direction */ 
  background-size: round auto; /* Height: auto is to keep aspect ratio */
  background-repeat: repeat;
}

This will adjust the image width so it fits a whole number of times in the background positioning area.

Additional note
If the size you need is static pixel size, it is still smart to physically resize the actual image. This is both to improve quality of the resize (given that your image software does a better job than the browsers), and to save bandwidth if the original image is larger than what to display.