jiff

Again, thank you very much @v_kyr. It's working now!

I think this is kind of “index printing” which even PowerPoint can do automatically.
The questions to be clarified:
how many images to process? (Per session / day)? all images have same aspect-ratio? Any manually pre-processing required? which export format, resolution? Single page, multiple files of single page, or single file with multiple  pages? Based on your answers, Photo or Designer is good for smaller number of images, and Publisher might handle very large amounts.
https://affinity.serif.com/en-us/tutorials/publisher/desktop/video/494072789/

That's nothing specific, pretty much the same as doing a template like a contact or photo sheet. - As an example for similar things see:
A contact sheet example

For Designer/Photo, dependent on the document size you need, aka if only for web display/preview or printing instead, setup a landscape oriented document the size/resolution you need with the default white background. Activate the rulers showup and grid etc. for your doc.
Now from the docs overall width value subtract the white gap sizes you want to keep and divide the remaining result by 3, so you know at first the horizontal width for each black rect image area to use. Do the same for the doc height, subtract the white area size to remain between image heights and divide the remaining result of that by 2, in order to get now the vertical height value for each of those image rects. - You can now drag out horizontal and vertical help lines from the rulers and place these help lines accordingly for layout and rect image area placement assistence.
Next create the 6 rect image areas via the help of setting their width/height values as input into the transform panel and layout/position them accordingly. If they are right positioned lock them in the layers panel so they can't be move accidently around. - Now save the whole as your custom template file!
When using that template, open it, place/load 6 images inside and clip them to the 6 image areas! - Afterwards you can place little round circles grouped with "a, b, c, d, e, f" or "1, 2, 3, 4, 5, 6) text inside on the clipped images.

Hi,
in addition: you can use the Guides Manager.

another pro tip:
create one folder (OS level, file system) withe the afpohoto file. Add 6 placeholder images to the folder (i1.png to i6.png). Place the images into the rectangles, like v_kyr explained. You can simply copy any new set of images into that folder (just keep the filenames).
If you open the afdesign file, it will be updated with the new set of images. If not, use the resource manager to manually start the refresh.
Then simply export the result.
If not, use the resource manager to manually start the refresh.
If you make multiple template folders (to cope with different number of images and aspect rations), you get a really fast process.
 

See here how to clip or mask a placed image layer to one of the 6 rects then:
Masking vs clipping layers (video tutorial)

Again, thank you very much @v_kyr. It's working now!

You can also draw your shape, then click 'Insert inside the selection' and then 'File>Place' your image and click and drag the image inside the shape - this method leaves you with an image you can still select and re-scale within the shape.