.wxml:

<view class="add-wrap"hover-class="plus-hover">
  <image src="/assets/plus_new.png"></image>
</view>

.wxss:

add-wrap {
  position:fixed;
  bottom:0;
  display:flex;
  width:100%;
  justify-content:center;
}
.add-wrap image {
  width:64px;
  height:64px;
}

效果：