Correction/clarification: I said that an image will be displayed at various sizes depending on the page, giving an event description vs an event calendar listing for example. This happens only if the image tag has NO height and width parameters in it. In that case, a very large image will scale up or down to fit whatever space is available to it in the layout. This is nice on a web page, but in my Outlook email display, it is HUGE, even though the type is normal size, and on (at least) an old iPhone some humongous images have mystified the display, unless size parameters are used, which, as I've just noted, misses the point.

My overall point remains.

Scaling down an image is relatively easy. I'm talking about changing the actual dimensions (in pixels) of an image, not just changing the size of its display (which is what the sizing of an image in WA's editor does).

There are various utilities for this online, but you (like me) might shy away from recommending this to your organizations' users, for security and standardization (training, etc.) reasons.

However, if you're using Windows 7 (the only current OS I'm familiar with), you can resize an image by using the "Paint" program that is part of Windows 7. (Start > Search programs and files > choose Paint. The results are entirely professional looking.

Sizing UP, however (and the main reason I'm posting this) is problematic. Unless your increase is small (roughly a maximum of 200%, assuming the original is sharp), the image will become blurry. This can sometimes be managed by using an image editor's "sharpen" tool, but without image editing expertise and suitable software, the results might not look so professional. Even so, you might like the effect, but I wouldn't rely on it as standard procedure.

You can also upsize the display of a small image by using the same HTML code that WA uses to downsize them. Assuming your photo is (for example) 100x200 and you want to make it show as 200x400, the code would be:
<img src="imagefilename.jpg" height=200 width=400>
However, a browser's upsizing algorigthm might not give you quite as good a result as upsizing it in, say, Photoshop, and definitely will make it more fuzzy the larger you go.

Test your results on a wide variety of devices, software, and versions. I realize that's what you're really asking for, and I'm agreed, it would be nice if the exact size image could be served depending on the device requesting it. To some extent, WA already does this: for example, you can insert a very large image, and size it to fit the small (about 375 pixels wide) event description workspace, but in the calendar display of all events, a much wider version will appear. Beware, however, that an image that is actually very large (e.g., 1200+ pixelseamight be slow to load or confusing to some small screen devices.

Compared with other things on my own WA wishlist, this is very low priority. For now, it might be better to educate your users on the advantages and artistry of well used "white space."