What image size should I use?
The answer about what image size should be used is both simple and complicated. If you are looking for the simple answer, then any proper image size is fine. Our system automatically adjusts images to the slides or layers, so if you just look at the slide, you can see about what size you should have and you don't have to overthink it, you can use an image around that size. Here is the complex answer why:
Your sliders are responsive, meaning they are adjusted to the width of the screen. A desktop screen size can be for example 1920px wide, but also 1200px wide. Big images can be made smaller and still look good, while small images cannot be scaled up too much without seeing them all pixelated. This means, that to support both 1920px and 1200px wide monitors, you could use 1920px wide images for the best quality. We suggest trying to support 1920px wide monitors and then your images will be fine probably on every screen they will be seen on.
Quality vs filesize
The better the image quality is, the bigger the filesize will be. Big filesizes can make websites slow, so you should aim for having the smallest quality what you find acceptable. This is the biggest question in this case, that what do you find acceptable? A photographer won't allow his/her images to lose any quality, even if the page loading speed will be slower because of it, but an average person doesn't need perfect images, as you can't even tell the difference in some cases, but you can tell the difference between the loading speed. For example going back to the support of both 1920px and 1200px wide monitors, you could use an image size somewhere between, like 1700px wide images.
In these scenarios we were talking about background images of fullwidth sliders. But you might aren't even using sliders with that size or the image in question is an image layer, which doesn't covers the entire slide. If you are using a slider, which is only displayed with maximum 1200px width, then don't use images bigger, than 1200px width. If you are using a layer, which only takes up 100px width, then don't use an image with bigger, than 100px width.
We haven't talked about image heights, because images can be displayed with many different ratios, in some cases you need bigger heights in some smaller heights. Slider sizes are affected by a lot of things and you can read all about them in here. The best we can suggest is to just test out one image to see its behavior. First you should pick an image size, which looks fine by default on your slide. If you want to hear an exact image size, then try 1920*1080px size and you could adjust that.