how to put an image inside a button? using html and css only

You could create the icon using CSS instead. This would allow you to apply transitions / animations etc.

.buttoni {
  width: 270px;
  height: 46.9px;
  position: relative;
  transition: .2s ease-in-out;
  cursor: pointer;
  border: 0;
}

.buttoni:before,
.buttoni:after {
  position: absolute;
  content: '';
  transition: .2s ease-in-out;
}

.buttoni:before {
  background: #29d4a2;
  border-radius: 50%;
  width: 25px;
  height: 25px;
  right: 15px;
  top: 10px;
}

.buttoni:after {
  width: 0;
  height: 0;
  display: block;
  border-left: 6px solid transparent;
  border-right: 6px solid transparent;
  border-top: 6px solid #FFF;
  transform: rotate(-90deg);
  right: 20px;
  top: 19px;
}

.buttoni:hover {
  background: #29d4a2;
}

.buttoni:hover:before {
  background: blue;
}

<button class="buttoni">Earn Money Helping <b>People</b> 
</button>

Well, if you want to do it using html

<button class="buttoni" style="width: 270px; height: 46.9px;">Earn Money Helping <b>People</b><img src="img/pla.png" style="width: 25px; height: 25.7px; margin-left: 220px; margin-top: -50px;"></button>

Do not call div inside button because it is an erroneous form, since the button supports the insertion of images by default

In case you want to do it using CSS

This should do do what you want, assuming your button image is 16 by 16 pixels.

.buttoni {
    background-image: url(/images/buttons/add.png); /* 16px x 16px */
    background-color: transparent; /* make the button transparent */
    background-repeat: no-repeat;  /* make the background image appear only once */
    background-position: 0px 0px;  /* equivalent to 'top left' */
    border: none;           /* assuming we don't want any borders */
    cursor: pointer;        /* make the cursor like hovering over an <a> element */
    height: 16px;           /* make this the size of your image */
    padding-left: 16px;     /* make text start to the right of the image */
    vertical-align: middle; /* align the text vertically centered */
}