Placing images in Grid in column order, not rows

The technique Eugene mentions, Gridify, is one of my favorite ID features!

In conducting a quick test, though, it seems that if you load the place cursor, then gridify while Placing the images, using the technique in the article, you'll get the images in a row/horizontal order. 

If you're finding that's the case, try this as an alternative:

1. Draw a graphics frame that fills the page.

2. Go to Window menu > Utilities > Scripts

3. Run the Script "Make Grid." A dialog will allow you to indicate the number of rows/columns you want on that page, along with the gutter widths.

4. Optionally, next select all the frames on the page and use Object menu > Fitting > Frame Fitting options to set them all to Fill Content Proportionally or whatever sizing you want.

5. Now, load your place cursor with the appropriate number of images and simply click down the column as you Place each image. 

This lets you control the order, and while not quite as automatic, is quite quick and easy to implement.

Check out the 'Gridify'

https://creativepro.com/exactly-indesign-tool/

Furthermore the gap between them will be equal to your column margins in Edit>Layout>Margins and Columns

Set this first to whatever gap you want - and you can make a grid of images down - up - left - right - in a grid.