I see now what the issue is.
You're looking at the thumbnail on your blog and you're not linking it to the original picture. My Dutch is pretty bad so you're going to have to figure out the translation when you do this.
Take a look at the FAQ on the uploading the picture suject and find where it says "You now have 6 options." What you're doing currently is the 'Using Thumbnail' and the 'Not Linked' options. (Again, for you it would be in Dutch.) What you're seeing on your website is the thumbnails which is why they're so small.
Two options here:
'Use Thumbnail' and 'Linked to Image' - That would keep the thumbnail within your Post but, when you click on the image, it would take you to the full size picture. This is what I normally recommend since there are still those out there on dial up connections.
'Use Original' and 'Not Linked' - This would actually put the full size image within the post instead of the thumbnail. I would recommend it for you because your pictures are technical in nature and folkd would be clicking through anyway and you probably don't want to interupt their reading.
Hope this helps,