If you drag a single image onto a Repeat Grid where all grid cells show the same image, the image will change in all the grid cells.
If you drag multiple images onto a Repeat Grid, you will set up a repeating sequence of different images. For example, if I drag in 5 images at the same time, the first 5 grid cells will show different images and then the pattern will repeat (e.g. every 5th grid cell will show the same image).
If you drag a single image onto a Repeat Grid that already has a repeating sequence of images, you’ll replace all copies of the image you dropped onto. For example, if you had a sequence of 5 images repeated 3 times (15 total grid cells), and then you drop a single image onto one grid cell, you’ll see 3 copies of that image since you’ve essentially just replaced one image in the master 5-image sequence.
I agree sometimes it seems more intuitive to replace just the one image you dropped on, instead of all repeated copies of that image. However, other times the current behavior described above seems useful too. How would you want to control or choose between those two different options?
Also, these two requests are related (basically more advanced supersets of this) and you may be interested in upvoting them as well:
If you drag a single image onto a Repeat Grid where all grid cells show the same image, the image will change in all the grid cells.
If you drag multiple images onto a Repeat Grid, you will set up a repeating sequence of different images. For example, if I drag in 5 images at the same time, the first 5 grid cells will show different images and then the pattern will repeat (e.g. every 5th grid cell will show the same image).
If you drag a single image onto a Repeat Grid that already has a repeating sequence of images, you’ll replace all copies of the image you dropped onto. For example, if you had a sequence of 5 images repeated 3 times (15 total grid cells), and then you drop a single image onto one grid cell, you’ll see 3 copies of that…
How does this need more information? Literally copy the way Sketch handles repeating symbols.