You're mixing notations. It should be:
<img src="folder/file.jpg" width="200" height="200">
(note, no px). Or:
<img src="folder/file.jpg" style="width: 200px; height: 200px;">
(using the style attribute) The style attribute could be replaced with the following CSS:
#mydiv img {
width: 200px;
height: 200px;
}
or
#mydiv img {
width: 100%;
height: 100%;
}