Photos not loading
Note : Protonmail seems to be the most sensitive to errors like this, so sending [test] emails to a protonmail address is a good way to test for errors.
Glitch noticed Apr 2022 : If photos look like they've been added to your email, but when you do a test email they appear blank with only the alt-text showing.... then check the file name.
It was found that file names with a + sign were being problematic.
Change the filename to something simpler. Add the image to the email again and do another test to yourself to check.
It is good practice to optimise your images within an email. Smaller files load faster and have a smaller carbon footprint! Also it has been noted that some email providers aren't loading large images. You can reduce the size of your graphics and images, preferably to 600px or 700px wide. You can either do this in graphics software before you upload OR you can do this after you've uploaded to your Action Network email. The instructions on this are on this page here.
Alt text glitch
Don't use quotation marks in the alt-text. This makes images fail to load in protonmail (and maybe other emails too)