You can modify the regular expression from /\.png$/ to /\.(png|jpg|jpeg|gif)$/ to accompany more extensions.
I don't think that making it decorative will make a difference. From the names, it sounds like there is a difference. Do not check that if it breaks accessibility. The file is still read from the system and that's what triggers the asset read.
One way that you could add things that they don't show up at all is to host them externally to Canvas. That then relies on another website being hosted and available, but if the images were available on a CDN on the cloud somewhere, it would not log them in the asset report. The asset report only logs what is inside Canvas.
Line dividers can be inserted using the Horizontal Rule <hr /> element. Then it's not an asset, it's just part of the HTML on the page, and won't get logged. You can edit the HTML with the Rich Content Editor, or in the New Rich Content editor you can open the menu (Alt+F9), go to Insert, and choose Horizontal Line.
Putting images into "my files" instead of "course files" will look right because you have access to them, but the students will not will likely see broken image icons or something similar. Do not do that.
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