When prototyping a new layout it's convenient to use placeholder images then it is to use the final images. You could make the placeholder images in an image editing program.
There is a far better way using a site called placeholder.com you can quickly get an image of any size you require by simply going to https://placeholder.com/ and placing your desired width and height after the address like: via.placeholder.com/350x150 which generates an image 350px wide by 150px tall you can then save the image and use it or use that path in an img src path like:
Which gives you this image:
You can also specify the colour of the background and text by passing them as extra parameters in the url call:
<img src="https://via.placeholder.com/350x150/226688/cfa" alt="" />
The first set of colours is the background the second is the text color.
A very useful service, one to bookmark.