best is at 800x600, any higher it will be pixelated.
rule is, if you have images lower than x800 or x1000 because of loss in detail, like most jpg. Therefor you can forget trying upsize it especially ones in color.
however it is different with line art with no shading
sorry i cant seem to upload the 800x600 image , cant do attachment either....so this is the link at 600x744
https://www.turboimagehost.com/p/88784988/800x600.jpg.html