The image path you put in the front-matter needs to be the relative path to the source directory of your website. For example, if you want to use the following image as a thumbnail:
```
<yourblog>/source/gallery/image.jpg
```
You need to use the following as the image path:
```
/gallery/image.jpg
```
Also, it is recommended that you put all the images under a dedicated asset folder that is separated from the `_posts` folder.