Why is it making a staircase with the images and not allowing them to sit by each other

[Jedijon]

.dropbtn {
    background-color: #4CAF50;
    color: white;
    padding: 10px;
    font-size: 16px;
    border: none;
}

.dropdown {
    position: relative;
    display: inline-block;
}

.dropdown-content {
    display: none;
    position: absolute;
    background-color: #f1f1f1;
    min-width: 800px;
    box-shadow: 0px 8px 16px 0px rgba(0,0,0,0.2);
    z-index: 1;
}

.dropdown-content img {
  width: 32px;
  height: 32px;
  margin: 3px;
  position: relative;
  float: left;
}

.dropdown-content a {
    color: black;
    padding: 12px 16px;
    text-decoration: none;
    display: block;
}

.dropdown-content a:hover {background-color: #ddd;}

.dropdown:hover .dropdown-content {display: block;}

.dropdown:hover .dropbtn {background-color: #3e8e41;}

/* HIDE RADIO */
[type=radio] { 
  position: absolute;
  opacity: 0;
  width: 0;
  height: 0;
}

/* IMAGE STYLES */
[type=radio] + img {
  cursor: pointer;
}

/* CHECKED STYLES */
[type=radio]:checked + img {
  outline: 2px solid #f00;
}

<div class="dropdown">
  <button class="dropbtn">Dropdown</button>
  <div class="dropdown-content">

<form name="form" action="icons-submit/submit_icon-default.php" method="post">
  <input type="radio" id="icon_default" name="icon_default"  value="icons/default.jpg" onchange='if(this.value != 0) { this.form.submit(); }'>
  <img src="icons/default.jpg">
<input type="text" id="id" name="id" value=" . $row['id'] . " style="background: transparent;border: none;font-size: 0;">
</form>

<form name="form" action="icons-submit/submit_icon-default.php" method="post">
  <input type="radio" id="icon_default" name="icon_default"  value="icons/default.jpg" onchange='if(this.value != 0) { this.form.submit(); }'>
  <img src="icons/default.jpg">
<input type="text" id="id" name="id" value=" . $row['id'] . " style="background: transparent;border: none;font-size: 0;">
</form>

<form name="form" action="icons-submit/submit_icon-Grass_Block.php" method="post">
<label>
  <input type="radio" id="icon_grass-block" name="icon_grass-block" value="icons/Grass_Block.png"onchange='if(this.value != 0) { this.form.submit(); }'>
  <img src="icons/Grass_Block.png">
</label>
<input type="text" id="id" name="id" value=" . $row['id'] . " style="background: transparent;border: none;font-size: 0;">
</form>
<form name="form" action="icons-submit/submit_icon-gear.php" method="post">
<label>
  <input type="radio" id="icon_gear" name="icon_gear" value="icons/Gear.png"onchange='if(this.value != 0) { this.form.submit(); }'>
  <img src="icons/Gear.png">
</label>
<input type="text" id="id" name="id" value=" . $row['id'] . " style="background: transparent;border: none;font-size: 0;">
</form>
  </div>
</div>

 

[requinix]

It'd be easier to tell by seeing the actual page but the problem is likely you floating elements.

Get rid of that on the <img>s and try this:

<div class="dropdown">
	<button class="dropbtn">Dropdown</button>
	<div class="dropdown-content">
		<form method="post">
			<button type="submit" formaction="icons-submit/submit_icon-default.php" name="icon_default[<?=$row['id']?>]" value="icons/default.jpg">
				<img src="icons/default.jpg">
			</button>

			<button type="submit" formaction="icons-submit/submit_icon-default.php" name="icon_default[<?=$row['id']?>]" value="icons/default.jpg">
				<img src="icons/default.jpg">
			</button>

			<button type="submit" formaction="icons-submit/submit_icon-Grass_Block.php" name="icon_grass-block[<?=$row['id']?>]" value="icons/Grass_Block.png">
				<img src="icons/Grass_Block.png">
			</button>

			<button type="submit" formaction="icons-submit/submit_icon-gear.php" name="icon_gear[<?=$row['id']?>]" value="icons/Gear.png">
				<img src="icons/Gear.png">
			</button>
		</form>
	</div>
</div>

I'm quite skeptical of those multiple PHP files you have there. What are they? They would need a minor adjustment to support the above markup.