Overlapping/overlaying multiple inline images

I like Temani's better, but if you can't use flex because you have to support IE 9 or earlier, I'll leave this here.

Note that the text direction is now right to left, so you'll need to reverse the order of your avatars.

.avatar img {
  border-radius: 50%;
  position: relative;
  left: -5px;
  margin-left: -25px;
  z-index: 1;
}

.avatars {
  direction: rtl;  /* This is to get the stack with left on top */
  text-align: left;  /* Now need to explitly align left */
  padding-left: 25px;  /* Same value as the negative margin */
}
<div class="avatars">
  <span class="avatar">
        <img src="https://www.fillmurray.com/50/50" width="50" height="50"/>
    </span>
  <span class="avatar">
        <img src="https://www.fillmurray.com/100/100" width="50" height="50"/>
    </span>
  <span class="avatar">
        <img src="https://www.fillmurray.com/200/200" width="50" height="50"/>
    </span>
  <span class="avatar">
        <img src="https://www.fillmurray.com/150/150" width="50" height="50"/>
    </span>
  <span class="avatar">
        <img src="https://www.fillmurray.com/50/50" width="50" height="50"/>
    </span>
  <!-- Variable amount more avatars -->
</div>

You can use flex and reverse order then no need z-index:

.avatars {
  display: inline-flex;
  flex-direction: row-reverse;
}

.avatar {
  position: relative;
  border: 4px solid #fff;
  border-radius: 50%;
  overflow: hidden;
  width: 100px;
}

.avatar:not(:last-child) {
  margin-left: -60px;
}

.avatar img {
  width: 100%;
  display: block;
}
<div class="avatars">
  <span class="avatar">
        <img  src="https://picsum.photos/70">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/80">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/90">
    </span>
  <span class="avatar">
       <img src="https://picsum.photos/100">
    </span>
</div>

Here is another idea with scale:

.avatars {
  display: inline-block;
  transform: scale(-1, 1);
}

.avatar {
  position: relative;
  display: inline-block;
  border: 4px solid #fff;
  border-radius: 50%;
  overflow: hidden;
  width: 100px;
}

.avatar:not(:first-child) {
  margin-left: -60px;
}

.avatar img {
  width: 100%;
  display: block;
  transform: scale(-1, 1);
}
<div class="avatars">
  <span class="avatar">
        <img  src="https://picsum.photos/70">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/80">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/90">
    </span>
  <span class="avatar">
       <img src="https://picsum.photos/100">
    </span>
</div>

Another idea using mask in case you want to keep the order of your images. This will also give you transparency between the images:

.avatar {
  display: inline-block;
  border-radius: 50%;
  overflow: hidden;
  width: 100px;
}

.avatar:not(:first-child) {
  margin-left: -60px;
  -webkit-mask:radial-gradient(circle 55px at 5px 50%,transparent 99%,#fff 100%);
          mask:radial-gradient(circle 55px at 5px 50%,transparent 99%,#fff 100%);
}

.avatar img {
  width: 100%;
  display: block;
}

body {
  background:pink
}
<div class="avatars">
  <span class="avatar">
        <img  src="https://picsum.photos/70">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/80">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/90">
    </span>
  <span class="avatar">
       <img src="https://picsum.photos/100">
    </span>
</div>

Another idea using a 3D tranformation trick (without transparency)

.avatars {
  transform-style:preserve-3d; /* here */
}

.avatar {
  display: inline-block;
  border-radius: 50%;
  overflow: hidden;
  width: 100px;
}

.avatar:not(:first-child) {
  margin-left: -60px;
  transform:rotateY(-1deg); /* and here */
}

.avatar img {
  width: 100%;
  display: block;
}
<div class="avatars">
  <span class="avatar">
        <img  src="https://picsum.photos/70">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/80">
    </span>
  <span class="avatar">
        <img src="https://picsum.photos/90">
    </span>
  <span class="avatar">
       <img src="https://picsum.photos/100">
    </span>
</div>