You can also change the size that images display using the plugin Joplin Enhancement
First, insert your image, and go to the markdown editor which will show, for example:

Now by adding, again for example, {width=50%}
, so it looks like this:
{width=50%}
the image will appear at half the original size (the image remains the same size it's just displayed according to what you've added in the curly braces)