Hi,
When you mouse over the image you want to change does the VEC blue outline wrapper identify that image as "image" or something else, like "container"? If it's container that likely means that image is a CSS background image. The VEC doesn't currently offer "swap image" on those types of images. You can still change out those images but you'll need to use another method, edit HTML on the parent element, or find the CSS selector and use a manual modification via the code editor. Something like this: