An image in a post being stretched to a square size by wordpress

Hi!

WordPress stretches images to a square size in my posts.
I upload an image, set it full size, no alignement, but in a post it appears to be forced to a square size.

Here’s a full code in a post:

<p><a href="https://www.robopark.eu/2010/wp-content/uploads/2010/06/kaart4.jpg" mce_href="https://www.robopark.eu/2010/wp-content/uploads/2010/06/kaart4.jpg"><img src="https://www.robopark.eu/2010/wp-content/uploads/2010/06/kaart4.jpg" mce_src="https://www.robopark.eu/2010/wp-content/uploads/2010/06/kaart4.jpg" title="kaart" class="alignnone size-full wp-image-46" width="680" height="454"></a><br mce_bogus="1"></p>

I’m using downloaded version of wordpress on rented server.
I’m using Rising Sun theme (modified):
https://www.ads-software.com/extend/themes/10pad2-rising-sun

How can I solve this? Im going crazy with this;)

Thank you very much!