To resize an image, click on the marked corner of that image and drag that corner out to make the image larger. Drag the corner in to make the image smaller. Release when you like what you see :) And don’t forget to hit save!
The marked corner for resizing is always on the bottom of the image. For modules that are just an image only, the marked resize handle is the right bottom corner. But for mixed Text & Image modules, the resize handle is on the bottom corner of the image that is closest to the text block. So, if the image is on the right, the resize handle is on the left bottom corner of the image. And if the image is on the left, the resize handle is on the right bottom corner of the image.
For a mixed Text & Image modules, the text will wrap around the image, automatically. So, to get a good idea of what your campaign will look like, as you resize the image, it's best to have the module in Preview mode. If you can see the border and text formatting tools in the module, you are in Edit mode. Just click the Preview button at the top of the module, to toggle to Preview mode.
The current pixel dimensions of the image, in your campaign, will display on the bottom corner of the image -- opposite the corner with the resize handle. And it will update live, while you are resizing your image. So, if you are trying to match several images to the same width or height, you can keep an eye on that dimension, and match the numbers you see.
Banner images are always the full width of your email, which is 590 pixels. Those can’t be resized. All other images can range from 79 to 554 pixels in width (height is not limited).