Does marking files as readonly make them secure?

Question

In Windows (or any OS, really), does marking a file as "Readonly" make it less susceptible to malware and unwanted manipulation? Additionally, should I still scan readonly files for viruses and other malware?

For example:

1. I download a wallpaper (.jpg) from sketchy.site.com.
2. Downloaded file contains malware that executes when the file is opened.
3. I mark downloaded file as readonly.
4. I open the downloaded, read-only file in my Windows Picture Viewer.

Will the malware embedded within the picture still be able to execute?

Same thing can be applied to any other file type. Most "readers" (text editors, pdf viewers, video players) only require read permissions, not Write and eXecute. Case in point: MS Office 2013 opens server / email attachments as readonly by default, for "security". (I'd post a screenshot, but apparently you need 10 rep to do that...)

Answer 1

In Windows (or any OS, really), does marking a file as "Readonly" make it less susceptible to malware and unwanted manipulation?

Absolutely Not; One of the first things randsomware malware does is attempt to change those permissions silently. This can be done on any operating system really.

Will the malware embedded within the picture still be able to execute?

Yes. The malware will be executed.