Good quality schematic exports are done by selecting SVG export or PS/PDF export. Both are vector graphic formats so they don't lose resolution if you zoom in.
SVG images are good for embedding in online HTML documentation.
Unfortunately graphs are not exported to PS/PDF or SVG. The issue is technical, spice generated graphs may contain 10000/ 100000 points. Exporting all the data to SVG or PS/PDF will just crash the viewers you want to use to see the images. Downsampling can be done but must be done carefully to avoid aliasing/artifacts. The png export is a bitmap image format and this exports graphs as well. Below an example of a svg export and a png graph. To see the svg download the file and open it with a browser or image viewer.