I don't know about the markup editor but you could open the screenshot in Preview and resize it there. Once in Preview go to the tools menu item and click on adjust size. I'm currently running High Sierra so I don't have access to the markup editor but if you can't do it in markup editor this method will work.
The markup editor doesn't have a control to resize screenshot. Instead you can skip clicking on the screenshot and let it get saved (which is by default on Desktop). Once it is saved, you can open it in Preview.app and use the Adjust Size... command under the Tools menu to resize it.
The way Instant Alpha works is that it selects color that are similar to it. It's not as nice as Photoshop where you can select multiple areas and get really granular with the selection criteria so you'll have to do multiple steps.
Select the Instant Alpha tool
Using your mouse, select an area you'd like to remove
Slowly drag your mouse down and select an ...
If you drag and drop onto the Printer queue window, or otherwise send a job to print without settings the print options, then CUPS will fall back on the default options for that queue.
Check the manual to see if there's a way to set the default for your printer using its control panel, or other method.
Failing that, you can set the default options for a ...
Giving the Apply guy the benefit of the doubt, there's likely a gap between what he meant and what he said and what was understood. Otherwise he's an idiot.
I can copy and paste text from PDFs that I have in Catalina; and Apple does indeed provide methods to select and copy text from PDFs in their frameworks for developers -- the same frameworks on which ...
What you need to do in Preview is to set the fill color of your image to white. You may have a border but it will be invisible. If your page color is black, then set the background fill as black too. You might need to set transparency to 100%. A little trial and error should determine this part.
It is possible to set the Border color and Fill Color of ...
The essential difference between Safari's Export as PDF and Save As PDF in the print menu is that the latter involves the print architecture, and the former does not. As mentioned in the comments, it's possible that old 32-bit print drivers may be causing problems in Catalina's 64-bit clean environment. Of course, this would be noticed when printing to paper,...
How can I change default font Notes - Sierra, High Sierra
Is this relevant? I think the same could be done for Preview but I don't know how to figure out the hexadecimal code for the font and font size I want.
I am struggling with the same problem and I really want a solution.
Even though this problem is old, I did figure out a solution that works (on Mojave at least, haven't tried it anywhere else).
Find a PNG image somewhere that already has a transparent background.
In Preview, remove the contents of that image and resize it to match your source image.
Copy the source image and paste it onto this (now blank) PNG image.