Hi Pepijn,
First of all, you can go to Backend > Extensions > Template manager > select JA Cagox template style > Layout tab > Responsive layout > Extra small
and Navigate to the position 1 & 2, you can resize each position so that it takes full-width in this extra small layout (mobile layout)
Refresh your site and see how it goes.
If image is still not resized, you then can create a new css file call custom.css in this directory: root/templates/ja_cagox/css/
and add this css rule:
@media (max-width: 320px) {
.singlecol.oddcol > img {
width: 100%;
}
}
Hope this helps.