Weleavefossils Glad you enjoyed the post! As for scaling down, if your resized image is jagged the there's a good chance you are using point sampling. When you downscale with linear sampling, there typically isn't any noticeable difference from the original - it's just smaller. You can check which you are set to in the project properties, which you can get to by clicking your project name in the "Projects" box on the top right of the C2 screen. Once there you should see this box on the left side of the screen where you can set your sampling type:
If you reference the concepts from the blog post, then I talked about an "original, high resolution image" to start with. Depending on the resolution of your game, you'll want to load in that asset, or at least one that is larger than what you need so you can test what image size you will need.
From there, you can resize the image inside of construct, or you can scale it. The difference here can be thought of in terms of permanent, or temporary.
When you resize an image from 1500x1000 to 150x100, then that image is now 150x100 - if you resize OR scale it back up to 1500x1000 the image will look terrible. Here's a couple images showing how you resize an image in C2 after double clicking your sprite - this is the only method you can use to resize your image inside of C2, everything else is used to scale the image:
When you scale an image from 1500x1000, that that image can be scaled to any size, but its still keeping the color information from the 1500x1000 asset, even if you scale it to 150x100 in your game, you can scale it right back to 1500x1000 no problem. You can scale the image with lines in the code, setting the numbers manually in the object properties box (when you select your sprite), or by dragging the corners of the sprite.
So, RAM and performance issues aside - for best visual fidelity, you should resize your image to the size you think will be the largest you will show that image in the game, depending on how the image is scaled (larger than 100% or smaller than 100% original size).
example: If you have a 150x100 asset, and scale it to 300x200, it will be very blurry. But if you have a 300x200 asset and scale it to 150x100, it will look fine.
However, having larger images in your game will cause it to use more RAM and impact performance more heavily, so you should try to find a balance. I find that you can typically scale an image to 120% it's actual size without much noticeable difference (won't be too blurry).
Hope this answered your question!
Also, you shouldn't use an image anywhere near 1500x1000 in your actual game, you should try to stay 512x512 or lower for the most part! I was just using some random assets for an example