Hey Natalia,
If you can create you own HTML and you did so from the beginning, I would continue to do so. WYSIWYG editors often insert unnecessary or erroneous code. I create all my HTML in just a simple text editor (e.g. notepad) then upload to Eloqua. At that point, you can edit in the Elq text editor, but for simplicity and ease of use at that point, I just copy my code out of Elq and paste it back into Notepad, make my edits, then paste it back in Elq and save. I am a little confused when you say "it restricts the view to the size of the text editor window." There are 3 options for viewing the source code editor, don't view, view vertically, view horizontally. The last two options give you the ability to slide either window to expand or contract your view of the code or visual. But regardless, I always pull my code out and edit in a fashion I am used to (i.e. Notepad, or for you, Dreamweaver).
As for field merges, I just drag those to the desired spot with in the actual code (not in the WYSIWYG window).
As for images, we use both Elq hosted images and images hosted elsewhere. The only concerning part about using images hosted outside of Elq would be if down the road that hosting was terminated and those images URL's were still in your Elq assets, they would no longer populate imagery. But if they are in Elq, they are always in the same spot as your emails and landing pages, so you can't accidentally send out messages with broken image links.