Hi Adam. I can share my experience with this, based on things I have tried.
I implemented a feature in my game (2D platformer) where the player will get a full-screen image when they examine certain things. So the player might examine a book on a shelf and they get a full-screen image of an open book and the text inside. The image is 1960 x 1120 - just slightly larger than 1080p. I have done it using a sprite object with 2 animations: 0 is empty and 1 is the image. It’s on the scene when the game loads and I just change the animation in an event i.e. I don’t create it or show it. It is always on the scene.
I haven’t split the image into pieces because it’s not going to be scrolling, unlike my main environments that I do chop into pieces of about 300 x 300 max (more on that further down).
I haven’t noticed a performance issue since I implemented this. The player can open and close the full-screen image and carry on with smooth scrolling afterwards. The image is about 150kb because I squeezed it down with a PNG optimizer.
I know it’s generally a no-no to have large images, as image guidance goes, but when it’s a relatively small filesize and it’s not animating or being created with an event, it seems to be absolutely fine. I’m not saying whether what I have done is a good way or not, but it’s working for me so far. My game, though, is for desktop PCs and not for mobile devices, and I imagine performance concerns with large images become much more pressing on mobile devices.
For scrolling levels, the cutting up of images in Photoshop and them putting them back together in GDevelop is time-consuming, for sure, but I get lovely smooth scrolling and no hitches by doing it that way. I have all the images in one object and change their animation on the scene to rebuild my artwork. I’d absolutely recommend this approach if you are not using tiles i.e. you are using a large piece of artwork for you level. It’s the difference between classic Mario (tiles) and Night in The Woods (large artwork). The latter approach gives you more creative flexibility, but does come with set-up drawbacks. I’d imagine, for example, dropping a large 4000 x 2000 image into GDevelop for a game level is going to cause issues unless you chop it up.
This thread might be of interest
And GDevelop wiki image guidance
https://wiki.gdevelop.io/gdevelop5/tutorials/reduce-size-game/#how-to-reduce-the-size-of-your-game