[Jellby]

Try adding this CSS:

.stroke img {
  width: 5em; /*or whatever is appropriate */
}

Or, if you have several images with different sizes:

<p class="stroke">
    <img src="Stroke1.gif" alt="Afbeelding" id="img1"/>
</p>

<p class="stroke">
    <img src="Stroke2.gif" alt="Afbeelding" id="img2"/>
</p>

img#img1 {
  width: 3em;
}
img#img2 {
  width: 5em;
}

PS. Do you actually want "Afbeelding" to show up when the image cannot be displayed? Think of text-to-speech, for instance.