If you're using Photoshop, I recommend using the "Save for Web" command. It's more powerful in several ways than the basic "Save As". It launches an interactive window where you can see a preview of the quality you'll be getting. It allows you to re-sample the saved image on the fly, changing the pixel dimensions to make the image a bit smaller, if necessary. When saving as a JPEG, you get the option to apply a very subtle blur to the image while saving, which can help iron out areas of high grain or noise that take a lot of MB to save. Essentially, the blur makes the JPEG compression more efficient (but don't use it unless you need it). When I use the blur, I do it at 0.3 or 0.5 pixels. For the JPEG quality setting, I find that below 65 makes the compression artifacts too prominent, while above 75 just bloats the file size without much visual benefit.

You also get a live preview not only of the appearance of your saved file, but also of the exact size in MB it will be. And none of these tweaks you're doing--not the blur, not the re-sample--will affect the actual image itself. They affect only the copy you're saving. By using Save for Web, you get this interactive experience of being able to precisely adjust the quality of the copy you're about to upload, without having to guess about it.

In my experience, the size cutoff for uploads to cartographer's guild is about 4.6 MB, give or take. I can usually squeeze a big map into something uploadable if I aim for 4.5 MB or so.