For all widgets of Matrix Themes with images, you need to host the file on your own website.
There are 3 ways to host your files on Jimdo website:
Navigate to design> Custom templates> files and upload you files for custom widgets images ( parallax, overlay teasers etc )
Once you upload your file, select it with right mouse over "copy link address" and paste it inside the widget/HTML like this:
<div class="is-matrix cc-clearover">
<img src="https://u.jimcdn.com/cms/o/s572cf8a914c2631f/userlayout/img/matrix-teaser.jpg?t=1405356750" alt="" /
The file folder is the most common way to upload the files for your widgets, however there's a limitation of a file size:
Please check the alternative 2 ways :
Create a hidden page where you can use the module "File download".
In view mode , select with right mouse over "copy link address" and paste it inside the widget/HTML
The images with big size you can host on your own website using design> background
Once added your image, you need to use developer tools and find the image url like this: