Gescom Hi there! It depends on your template. In case you want to develop your own theme, you can enable thumbnails for only .png and .jpg and use the original URL for .gif.
<@ if @{ image | match ('/(jpg|png)$/i') @>
<@ with @{ image } { width: 400 } @>
<img src="@{ :fileResized }">
<@ end @>
<@ else @>
<img src="@{ image }">
<@ end @>