First off, welcome to MR!
Let’s break down your question a little bit.
- 375 pages of text shouldn’t be anywhere close to 6MB of data. I think you may have a huge amount of code bloat (unneeded html/css). I would expect a book that size to be about .5MB. For comparison, I am combining all 3 of the Lord of the Rings into a single file and the text only is about 1.5MB.
- the whole point of an ePub is that it is a self contained file… it has everything it needs (text, images, font, styling, etc.) in one location so it can be read anywhere. Think about the individual that is reading the book while on a boat/airplane, or in the mountains, or after a weather induced power outage, and they don’t have Wi-Fi. Trying to read a book like that would cause them to report your book as 'broken'… not to mention just giving them a bad experience.
I would recommend processing the images so that they are the smallest file size you can get while still maintaining clarity. Different file types do much better than others depending on the type of image (.png, .svg, .jpg) are commonly supported.
If you still have a file size that is too large you can consider breaking the file down into smaller sections/divisions/volumes.
- if you really must go down the path of external resources then, assuming the server is maintained forever (domain name/address is maintained) and the data isn’t archived, you can reference an external source with the proper html link. This question has been asked a few times so just try a search of the forum.
The short answer is just to give the proper location of the image in the <a> tag.
Here is a quick tutorial on hyperlinks.
Example:
<a alt="mountain scene of lake in valley" src="yourserverdomainname/yourfolder/yourimage.jpg"/>