These are the steps you need to follow: Start by placing a full width structure bloc on the page. Add a modal bric - it will automatically adjust to the full width of the containing column.
With the Modal selected, create a custom class and make the following adjustments in the class editor:
By setting a size and then having the left and right margins set to auto, your modal will always remain in the centre of the column.
Your modal now has a fixed size so it will remain the same size regardless of content.
To style the footer, select modal footer as shown below and creat another custom class. In the paintbrush tab, set the background colour you want and any borders.
Next add your content (buttons, images text etc). Make sure that everything sits within the modal. When you place an image, you should adjust its width so it looks good in the modal. The idea being to ensure that the height of the image doesn’t extend into the footer area.
Once your actual image has been placed in the image placeholder, you can adjust the position of the final image by creating another custom class:
When all is well with your first modal, duplicate it as shown below:
You will now have a number uniformly designed modals - all the same size and shape.
Replace the content on your duplicates. Be sure to make all your images the same size before adding them to the modal. If your images are all different orientations or sizes you will have to add more custom classes to each individual modal to get the images to fit within the available space. Personally, I would set a standard orientation and size and then use an image editing app to create uniformly sized images. This may mean that portrait images have to be centred within the image canvas creating white space on either side, but this is a minor issue. You can always link your images to a lightbox so that the original full size image can be displayed if the user wishes to.
Finally, switch to your other breakpoints and adjust your custom classes to better suit the other device screens. Remember, because you have used the same custom classes on all your modals, adjusting those classes on other breakpoints will apply the changes to all the modals.