Faccia

Thanks again to all of you for your time, effort and experience in trying to identify the issue. 
@Dan C
I'm using Mac Mini M1, 2020
macOS 13.4 (22F66)

On top of the advice above, this sort of thing can happen if you “download” an image from a website incorrectly.
Sometimes, when an image with transparency is displayed on a website (e.g. a stock images site), the image is shown on the page with the chequerboard to show you that the original image has transparency. If you try to save that image directly from the page – by right-click and Save, or some other similar method – what you will get is the image including the chequerboard (the displayed image). The correct method is to use the ‘download’ button (or whatever option is offered) on the page to get the original image rather than the one you are shown on the page.

I assume you copied the image from the internet. Many times we search for images and add the extension .png to find those that are transparent, but the image, although it has that extension, is not really transparent. Don't think twice, the image is not transparent and the checkerboard background is part of that image. This is not your or Affinity's mistake.

I kept on dragging the guide to the ruler to remove, which like others have said is very common way to remove them in other programs. When I zoomed out, I then saw a collected mess of guide lines that just didn't want to go away. It's such a pain the way it is designed. I'm surprised Affinity has not changed this given that this thread dates back a few years and it clearly hasn't changed.

How very strange that this works for you and not me. I just tested in all three apps, and the behaviour is the same for each—if I'm zoomed in on the page and quickly drag and drop a guide to (or beyond) the ruler, the guide just ends up off screen at the point where I released the mouse button.
 
This was mentioned earlier, in response to the original question. At this stage we're discussing how the task of deleting guides could be improved. If you happen to be zoomed up on a page, I think you ought to be able to drag a guide back to the ruler to remove it. That just makes intuitive sense to me.
You know, I just assumed this is how it worked in Illustrator, but I just checked and no—it only works that way in InDesign! I guess in Illustrator, I was just in the habit of selecting a guide and hitting delete.
I think InDesign gets it right in every way. You can delete a guide by any of these means:
Drag a guide to the ruler Select a guide and press the Delete key Select Layout > Create Guides and check 'Remove Existing Ruler Guides'. This just makes it all super easy. In addition, you can place guides off the edge of the page should you wish to—although the value of that might be debatable. You can also trigger auto-scroll by dragging a guide off the page and beyond the ruler—although the value of autoscrolling for guide placement is also debatable IMHO.

Yes, or drag it back to the ruler from whence it came (which is how I've always removed guides in other apps). Given that this doesn't work, dragging it off the canvas always leaves me wondering if it's really gone, or if it's just hiding out of sight.

I agree with Max M, it would be useful to be able to change the preference for saving a JPG that was opened and quickly edited (eg. crop) at a level below 100%. I also understand that the default is set to 100% but it would be great to be able to modify that. I also understand that JPG files are not meant to be working files but sometimes its all you have to work with.
In other words, it would be great to be able to open a JPG, do a quick crop, and resave the file in the new size without having the size of the file double. I know that one can export the file and then change the quality of the JPG there but like Max M mentioned, its more steps and time consuming,

Lets bump this as it is a small thing that would help work flow and merely resets to the defaults

Gets my vote. Sensible is as sensible does. You can't argue with a one click reset function.

If anyone uses the clone brush a lot and is constantly changing the angle, scale or flipping the selection, you might agree with having a button to reset the brush back to defaults