You're welcome, Robbie.

As for the file format, if it is just for posting, like in the Community, I usually use Mac built-in screen capture app which automatically saves in png format with the default resolution(which depends on the monitor, ex: doubled in retina display). I usually trash them once posted. I don't use JPG since it is in nature of lossy compression. I would recommend to use PNG or BMP which has lossless compression or without any compression to avoid aliasing.

To use plots for publication, I right click on the Mathematica plot, and select save graphic as adobe pdf files, which are high resolution files. Then to prepare them for publication, I open the pdf files in Adobe Photoshop with the resolution set higher than the default of 300 pixels per inch on my computer. I usually use 600 or 1200. Then I do my alterations, this results in layers that I save as psd files, in case I have to redo them, which is often the case. To make the output file, I flatten the image into a single layer and then use save as tif with LWZ compression which makes files as small as jpg files but are lossless and much better quality for line drawings especially, and which most publishers accept, indeed, many prefer. This procedure results in small files with quality as good as needed. I am sure there are lots of other way of creating good graphics, but the way I do it is similar to what a graphics artist would choose, at least some of them.