The dialog box below will appear upon right-click > Show pictures (HTML only) until you tick the "Do not show me this warning in the future." option. For the life of me I can not figure out how to re-enable the display of the dialog box.

In a second Pegasus Mail installation the dialog box appears when both IERenderer and bearhtml are disabled which indicates it is a Pegasus Mail function. The setting is not identity specific but is user specific so the option must be stored somewhere in the mailbox directory. Logically, PMAIL.INI should hold the key but a file compare with a backup did not identify it. I then did a file compare of all files that were modified after I ticked the box without being able to identify an applicable entry.

Obviously I am missing something. Does anyone have a clue, or care to try your hand at figuring it out? Keep in mind that once you tick it you may not be able to go back.

This line with the 2 in PMAIL.INI has it suppressed
Integer preference block 1 = 12319 24000 1 0 2 0 0 7 0 0 0 0 0 0 0 0
This line with the 0 has it re-enabled
Integer preference block 1 = 12319 24000 1 0 0 0 0 7 0 0 0 0 0 0 0 0

Copied the contents of a test users mail directory, and then changed settings.
Comparied all files to see which actually changed, and PMAIL.INI was one.
Then used diff to see what lines withing changed.

Thanks @msetzerii! I wonder why my file compare didn't detect this.

To be clear, there can be numerous "Integer preference block 1" entries in PMAIL.INI. There is one in the [Pegasus Mail for Windows] section and one in each identity section. It is the one in the [Pegasus Mail for Windows] section that we are talking about.

In my case, the entry was this:
Integer preference block 1 = 159 24000 2147483647 0 2 0 0 7 1 0 0 0 0 0 0 0

Changing the fifth segment from a 2 to a 0 as per below reenabled the display of the dialog box.
Integer preference block 1 = 159 24000 2147483647 0 0 0 0 7 1 0 0 0 0 0 0 0

It appears that "2" and "0" are not the only options for the fifth segment. One of my installations has a "10" in that segment. I last used it in offline mode for testing so the "10" might be a way to disable the 'Show pictures' function when offline. I did not test this theory because I don't want that installation to ever run online.