I made an experiment and I think the problem is on GMail side.
When you write an email in a web-browser, such as when using GMail, you’re composing an HTML document, even if this is hidden through a UI with buttons for bold, italics and all other formatting. In such a document, pictures are described in an <img> element with a link to a file whereas in Writer, unless you specifically requested it otherwise, pictures are embedded with all the data in the document (Writer is clever enough to decode image data and display it).
Apparently, either the browser understands data passed by Writer (several representations are created and handed over to the browser) or Writer already has translated the document into HTML.
I even tried to create an HTML document with images directly in Writer and to paste it into GMail but this does not change the result: images are just rendered as placeholders.
IMHO, The paste process is unable to cope with multiple source files. Document text is pasted into the browser which translates it into “classic” HTML but images needs to be uploaded to GMail servers. Apparently the scripts in GMail can’t do it simultaneously with text conversion.
When you paste a single image, you’re only processing some graphical element and the scripts do the job of uploading image data, receiving from the server the uploaded filename (only the remote filename is meaningful in an email; your local filename can’t be referenced) and insert this filename into the <img> element.
If you want to avoid the painful manual update task image after image, adopt another procedure for sending formatted mail.
My best advice is to go for a real mailer application. Even if your group policy forbids attachments, you could imagine granting privileges to an “administrator” or “mail master” so that this person can send attachments to other members.
EDIT
I tested pasting into Thunderbird. Images are pasted simultaneously with text. But the document ends up in HTML, which means all the “beautiful” formatting you have designed is messed up. Inter-paragraph spacing is lost, as are indents. It also looks like line spacing is set to some fixed value making lines too close to each other. Font size variations are also lost. As the document is now HTML, there are no longer any header or footer, nor margins. Lines extend as far as the window allows and are reflown when you change window size.
If document formatting is important for you, read my advice about attachments.
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